Activities of delta-aminolevulinic acid (ALA) synthase and ALA dehydratase, the first and the second enzymes of the heme biosynthetic pathway, were determined in erythroid colonies derived from normal human bone marrow in culture. ALA synthase and ALA dehydratase were induced when normal human bone marrow cells were incubated in the presence of erythropoietin (Ep) in culture, and the extent of induction of ALA synthase as well as ALA dehydratase was dependent on Ep concentrations and culture periods. Both ALA synthase and ALA dehydratase were increased in cultured erythroid cells before the appearance of morphologically recognizable hemoglobinized colonies, corroborating the sequential induction of heme biosynthetic pathway enzymes in human bone marrow erythroid cells.