We have discovered that human and rabbit bone marrow conditioned media (BMCM) promote the growth of bursts in cultures containing erythropoietin. We then demonstrated that the measurement of 59Fe incorporation into heme was a more quantitative assay for burst-promoting activity (BPA) than counting burst numbers. Furthermore, we have observed that the use of low (less than 15%) concentrations of fetal calf serum is a convenient way of reducing endogenous sources of BPA for studies of the BPA in the test samples. When 59Fe incorporation and cell numbers of individual rabbit erythropoietic bursts were analyzed simultaneously, BPA caused a shift in the cumulative frequency distributions without changes in their shapes. These results indicated that BPA augments hemoglobin synthesis by stimulating cell proliferation during the early phase of burst formation. In addition, human BPA increased the relative proportion of fetal hemoglobin in the hemoglobins synthesized by adult circulating BFU-e. BPA appears to augment cell proliferation of early precursors which are committed to produce F cells.