In an attempt to predict the clinical response to androgens, the effects of added fluoxymesterone (FMT) on the in vitro growth of marrow erythroid progenitors (CFU-E) were examined in 15 patients with aplastic anemia and 6 control subjects with normal marrow morphology. In the control group, the addition of FMT enhanced the growth of CFU-E in vitro, the maximum absolute increase being dependent on the basal number of CFU-E. In 10 out of the 11 aplastic anemia patients who responded to androgens in vivo. FMT enhanced the growth of CFU-E in vitro. In this group, there was a positive correlation between the basal number of CFU-E and the maximum absolute increase of CFU-E induced by FMT. In patients who did not respond to androgens in vivo, all except one showed no increase in the growth of CFU-E in vitro by the addition of FMT. Furthermore, in four out of seven patients examined prior to androgen therapy, the addition of FMT enhanced the in vitro growth of CFU-E. Androgen therapy in vivo resulted in marked hematological improvements in all of these four patients. Two out of the remaining three patients in whom FMT did not enhance the in vitro growh of CFU-E failed to respond in androgen therapy. These results indicate that an in vitro CFU-E culture system to the presence of FMT may be helpful in predicting the response to androgens in vivo in patients with aplastic anemia.