We have investigated the effect of PGE 2 on the growth of human blood CD34 + hematopoietic progenitor cells. Cultivation of cells in liquid culture for 3 days with PGE 2 (100–10 nM) stimulates their [ 3H]thymidine incorporation into DNA. Semisolid cultures of these cells reveal the growth of an elevated number of BFU-E and CFU-GM colonies as compared with controls. Addition of PGE 2 (10 nM) in methycellulose cultures immediately after purification results in an elevated cloning efficiency of both BFU-E and CFU-GM. Finally, by using limiting dilution assays we demonstrated that PGE 2 acts directly at a single cell level. These results indicate, for the first time, that PGE 2 may enhance human erythropoiesis and granulopoiesis by a direct effect on human CD34 + progenitor cells.