These studies were undertaken to evaluate the effect of Calusterone (a weakly androgenic steroid) on hemopoiesis in mice. Cellular proliferation was suppressed by a single (IP) injection of busulfan (BU) (40 mg/Kg). Calusterone (CA) was administered s.c. SC daily (10 mg/Kg); controls received an equivalent injection of oil vehicle. Hemopoiesis was characterized by measurement of peripheral blood neutrophils, bone marrow cellularity, differentials and stem cell content. This included pluripotent (CFU-S), granulocytic (CFU-C) and erythroid (CFU-E) progenitor cells. Only a minimal decrease in narrow cellularity was observed after busulfan; similar values were obtained in calusterone recipients. Neutrophils fell by day 4, showed an abortive rise on day 8 and subsequently fell to 32% of control values. Calusterone recipients showed a 2 fold higher value (62%) on day 12. CFU-S, CFU-C, and CFU-E were depressed to 20-40% of control values by day 2 after busulfan. Although CFU-S and CFU-C remained depressed through the 14th day, CFU-E recovered by day 8 CA stimulated an overshoot in these cells to 288% of control values. These findings correlated with an increase in marrow erythroid cells to 182% on day 10. CFU-S remained low (20%) by day 14 and gradually increased to 50% of control by day 24. A delayed 10 day course of CA more than doubled the CFU-S recovery. These findings show that BU markedly suppress hemopoietic stem cells: a differential recovery is noted between CFU-E and the other progenitor cells. CA increase the recovery of all 3 hemopoietic stem cell compartments when given either immediately or in a delayed schedule. This suggests that this compound may be of use in the therapy of bone marrow hypoplasia.