The comparative influences of phytohemagglutinin-stimulated leukocyte conditioned medium (PHALCM), hemin, prostaglandin E1 (PGE1), and growth of cells at low oxygen tension (5% O2) were evaluated for their capacity to enhance colony formation in vitro from normal human bone marrow erythroid progenitor cells (BFU-E). Each treatment enhanced colony formation by itself, and the combinations of treatments resulted in an additive enhancing effect on erythroid colony formation. Removal of T-lymphocytes from the bone marrow sample ablated the enhancing activity of PGE1, but did not influence the enhancing activities of PHALCM, hemin, and growth at low oxygen tension. The results suggest that the mechanisms of action of these various erythroid colony-enhancing effects may be different.