Milan Hasek and the discovery of immunological tolerance.

This year is the 50th anniversary of the publication of a paper by Milan Hasek, in which he showed the phenomenon of immunological tolerance by the selective failure of chimaeric chick-embryo parabionts to produce antibodies against the red blood cells of each other. The discovery of tolerance was credited by the Nobel prize, but excluded Hasek, because he misinterpreted his original experimental results. Hasek exuded an impressive personality and a much admired joie de vivre. With the benefit of hindsight, this article assesses the background of the period and the circumstances that led to this important discovery. I discuss Hasek's experimental ingenuity, the influence of the Lysenkoist genetic doctrine, Hasek's acceptance of the immunological theory to explain his work and his role in establishing a successful institute at the Czechoslovak Academy of Sciences.