To determine whether the CFUE pool within human bone marrow is heterogeneous with respect to erythropoietin (Epo) requirements, we studied CFUE growth in vitro as a function of Epo exposure time. We controlled the Epo exposure time by overlaying plasma clot cultures containing 0.5 U/ml Epo with an antibody prepared against human Epo. All cultures were grown for seven days. Benzidine-positive colonies containing eight or more cells were counted as CFUE. Colonies containing eight cells required at least 24-50 h of Epo exposure, and those containing 16 cells required at least 70-90 h of Epo exposure. After 50 h, the number of CFUE progressively increased until it reached a maximum at seven days of Epo exposure. These results suggest that the CFUE pool within human bone marrow is heterogeneous with respect to Epo requirements.