It is perhaps not too much of an exaggeration to claim that experimental hematology as it flourishes today originated largely from the pioneering attempts to protect lethally radiated animals (1) by shielding of hemopoietic tissues by L. O. Jacobson (9), and (2) by treatment with bone marrow suspensions by E. Lorenz and his col- laborators (12). The site chosen for this annual meeting of the International Society for Experi- mental Hematology is given a special historic sig- nificance by the fact that it was 25 years ago that the first publication on this subject by Lorenz ap- peared from his laboratory at the National Insti- tutes of Health. Lorenz's discovery marked the beginning of a period which lasted until 1956, during which the protection afforded by hemopoietic cell suspensions was confirmed by many. This soon led to an intensive scientific de- bate on the mechanism of this protective effect: was it due to a humoral factor produced and pro- vided by the bone marrow-as Lorenz The Appearance of postulated-or to transplantation and subsequent proliferation of hemop- etic cells? This question was defini- 1 the Multipotential tively answered in 1956 by evidence from three different laboratories (7, 15, 26), which demonstrated the origin of the cells Hemopoietic in the repopulated tissues using a variety of cellu- lar and immunologic markers. By the same token, these contributions marked the birth of radiation Stem Cell chimeras.