The efficacy of in vivo administration of a 5 alpha-androstane and two 5 beta-androstanes (3 alpha and 3 beta) on CFU/GM and CFU/E mouse bone marrow stem cells was compared. The subjects were two groups of Swiss Webster mice: one normal group and one group in whom chronic aplastic anemia was induced by irradiation followed by mesenteric node lymphocyte grafting from C57/Bl donor mice. In normal animals, the two types of androgens (5 alpha and 5 beta) had the same efficacy, and the injection of 5 beta-androstane, before that of 5 alpha, significantly increased its efficacy. In aplastic mice, 5 alpha and 5 beta-androstanes had the same efficacy on CFU/E but 5 beta-androstanes were more efficient on the granulopoietic committed stem cells. The efficacy of the association of the two compounds in aplastic mice was not superior to that of each drug alone, which can be explained by the depletion of the stem cell compartment induced by the stimulating effect of the first androgen.