Highly purified human blood burst-forming units-erythroid (BFU-E) were used to study the effects of interferon gamma (IFN gamma). IFN gamma inhibited erythroid colony formation, cell proliferation, and differentiation of day 3 to day 6 mature BFU-E in a dose-dependent manner. The primitive BFU-E (day 1 and day 2 cells) and later day 7 cells were less affected. IFN gamma dose-response experiments demonstrated that the number and size of erythroid colonies were reduced at a concentration of 500 U/ml with more complete inhibition at 1,000 U/ml. Inhibition of day 4 to day 6 erythroid progenitors was first noted by 72 h of incubation with IFN gamma, and target cell growth and differentiation continued to decrease with further incubation. IFN gamma also induced erythroblast apoptosis which was demonstrated by both nuclear condensation and fragmentation plus flow cytometry with in situ end-labelling. Because day 3 to day 6 cells need stem cell factor (SCF) for development in serum-free culture, the relationship of IFN gamma inhibition to this growth factor was investigated. The reduction in the number of erythroid colonies by IFN gamma was reversed by SCF although the colony size was not completely re-established. In contrast, interleukin-3 did not have the capacity to overcome the inhibitory effects of IFN gamma. Since IFN gamma blood levels are elevated in some anemias of chronic disease, IFN gamma may have a role in promoting this anemia and its inhibitory effect might be better overcome by SCF plus EP. However, the mechanism by which these growth factors overcome the inhibition of IFN gamma, or vice versa, is unknown at the present time.