There is no strain of humanism that was ever more intellectually sophisticated than the Marxist humanism generated by Eastern European intellectual dissidents, and, independently in many instances, anti-Stalinist Marxists in the West. Now I want to focus on the East Europeans, who entered into a symbiotic relationship with western humanists. (The Wikipedia articles are not perfect, but they are convenient entry points.) Most influential were the members of the Praxis School in Yugoslavia and various philosophers in Czechoslovakia, Poland, and Hungary.
The documentary record of interaction between western and East European anti-Stalinist Marxist humanists (note that the Stalinists and orthodox Communist Parties also called themselves humanists) is all over the place. There is one book on the subject I need to track down:
Tolerance and Revolution: A Marxist-non-Marxist Humanist Dialogue, edited by Paul Kurtz and Svetozar Stojanovic. Beograd: Philosophical Society of Serbia, 1970.
In the interim, I began to research Paul Kurtz's interaction with the Marxist Humanists on the web. I have found this most interesting and my picture of Kurtz is slightly altering in the process. Kurtz apparently flirted with the left in the 1930s and seeing what Stalinism had wrought, became a mainstream liberal in the Cold War period. In technical terms, he is best classified as a social democrat, which is the more advanced European equivalent of what Americans called liberalism from FDR's New Deal up to LBJ's Great Society. Kurtz's age matters, for his memory reaches back more than eight decades, and can gain more sympathy with his personal philosophical orientation (apart from his functioning in an institutional capacity) from reading his reminiscences. I will begin with some samples in this post and continue in future posts.
Secularism and Religion in America by Paul Kurtz
I am happy to return to Yugoslavia. This is my fifth visit. My first was in the mid 1960s when my wife and I drove as tourists from Italy to Zagreb in Croatia on a sight-seeing expedition. The second was on the occasion of the first Marxist non-Marxist Humanist dialogue, held in Montenegro, in Herzeg Novi on August 11-16, 1969. This dialogue followed an earlier open dialogue in Vienna in 1968 at the World Congress of Philosophy on a similar theme.
The Herzeg Novi dialogue was sponsored by the Yugoslav Philosophy Association, the Serbian Philosophy Association, and the International Humanist and Ethical Union (IHEU). Participating in that dialogue from Yugoslavia were Svetozar Stojanovic, Stanisa Novakovic, Mihailo Markovic, P. Vranicki, and Ljubomir Tadic. There were participants from the United States, Germany, Belgium, France Great Britain, Italy, The Netherlands, and from Czechoslovakia and Romania in Eastern Europe. 
A second Marxist/non-Marxist humanist dialogue was held at Boston University in Cambridge, Massachusetts in 1970; a third dialogue in Dubrovnik in 1973, and a final fourth dialogue, again in Dubrovnik, in 1979. At these dialogues we discussed tolerance, human rights, self-management, and democratic participation. They were important because they helped crystallize an intellectual and democratic opposition to totalitarianism in the former Soviet Union and Eastern Europe, and in a modest way they contributed to the eventual overthrow of dictatorships.
The Praxis Group of Eight philosophers were under constant fire from Tito, who needed Western support in his conflicts with the Soviets. Many people thought Yugoslavia was the most liberal Eastern European country because it permitted some degree of dissent. We in the West supported the Praxis philosophers and provided a constant barrage of letters and press releases to the Western press on their behalf. We thought Sveta Stojanovic was especially courageous for his heroic stance against repression and for democratization. The socialist humanists of Eastern Europe at that time pointed out the contradictions between socialist ideals and reality. They focused on the early Marx in order to defend the principles of humanism. But this is all past history.
 The papers of this conference were published in Serbia in a book here entitled Tolerance and Revolution, edited by Paul Kurtz and Svetozar Stojanovic (Philosophical Society of Serbia, Beograd, 1970). Incidentally, this was the very first book published by Prometheus Books, which had just been founded in the United States. Prometheus has since published some 2,500 books and has become a major publishing company.
Note the essay "Humanism and the Freedom of the Individual" in Toward a New Enlightenment: The Philosophy of Paul Kurtz, by Paul Kurtz, edited with an introduction by Vern L. Bullough and Timothy J. Madigan (New Brunswick : Transaction Publishers, 1994), pp. 49-62. "This chapter was originally delivered at the Marxist-non-Marxist Humanist Dialogue held on September 6-7, 1968, in Vienna, at a meeting of the World Congress of Philosophy. Published in The Humanist (January/February 1969), and In Defense of Secular Humanism."
In Defense of Eupraxophy by Paul Kurtz: Kurtz analyzes the failure of Soviet Marxism-Leninism and Soviet atheism. While he judges Marxism a failure in practice, he nonetheless states:
After a century of Marxism—and Marx was no doubt the greatest humanist thinker of the nineteenth century—and after the patent failure of Marxism, the question can now be raised, Where does atheism now stand?Furthermore:
Humanism must address itself to the heart and the passions; it must have some relevance to practice and conduct; and it must have some effect upon how we live. I submit that broadly conceived the freethought movement has failed in that direction. Marxism was an effort to apply humanism to practice, and indeed Marxsaid that atheism was merely abstract, that it only became meaningfully expressed when it was realized in terms of Communism; and so Communism offered a program and an agenda for the future liberation of mankind. The Marxist-Leninists failed because they developed a new tyranny. And so we now see that Marxism without freedom is not an authentic humanism. But we must not give up on Marx's basic insight that humanism only has meaning if it is related to practice.And here is the concluding paragraph:
We need to step up to a new plateau, and that, I submit, must be a plateau that defines a new eupraxophy that is relevant to the human condition, can inspire human beings to commitment and action, and provide meaning to their lives. This task is all the more pressing given the apparent collapse of Marxism, and the great vacuum in the world for inspiring ideals. Unless an authentic, democratic, scientific, and secular humanism can be identified as a viable alternative, then we may again be threatened by a new outburst of orthodox theism, and new cults of irrationality are most likely to emerge to plague humankind.The Secular Humanist Prospect: In Historical Perspective by Paul Kurtz, in Free Inquiry magazine, Volume 23, Number 4: Kurtz traces the rise and fall of humanism around the world. Kurtz identifies six major ill-boding changes since the 1970s contributing to the decline of humanism. Note:
The third factor that emerged to challenge freethought and the secular movement was the near-total collapse of Marxism. For a good part of the nineteenth and twentieth centuries, Marxist-humanist ideals had influenced intellectuals; with Marxism’s eclipse, anticlericalism and indeed any open criticism of religion have all but disappeared.