Effects of the immunosuppressants, FK506, deoxyspergualin (DSG), and cyclosporine A (CsA) on the growth of human hematopoietic progenitor cells were tested in the presence of interleukin-3 (IL-3) with purified bone marrow and blood cells as targets in methylcellulose culture. FK506 had a significant stimulatory effect on the growth of colony-forming units/granulocyte-macrophage (CFU-GM) and burst-forming units/erythroid (BFU-E) from peripheral blood and cord blood cells but not from bone marrow cells. Neither DSG nor CsA had an effect on any type of target cell. Liquid-suspension-limiting dilution assay with IL-3 showed that FK506 directly stimulated the growth of blood progenitors in a dose-dependent manner with single-hit kinetics. Liquid-suspension preincubation of blood cells with FK506 before culture in methylcellulose induced a significant increase in the amount of IL-3-supported growth of CFU-GM and BFU-E, whereas initial preincubation with IL-3 and subsequent culture with FK506 plus IL-3 exerted its stimulatory effect only on BFU-E. These data suggest that the stimulation of hematopoietic progenitor cells by FK506 occurs at a very early stage of maturation and diminishes with further myeloid development.