The effects of the tumor-promoter phorbol myristate acetate (PMA) on normal hemopoiesis and Friend leukemia virus (FLV) granulocytic leukemogenesis in long-term bone marrow cultures were examined. FLV- anemia-inducing strain (FLV-A) infected, Rauscher R-MuLV clone M52R infected, or uninfected control NIH Swiss mouse marrow cultures were treated weekly with PMA or 4-O-methyl-PMA at 2.0 ng/ml or 200.0 ng/ml. Addition of PMA to control uninfected or R-MuLV-infected cultures decreased production of nonadherent granulocytic cells and granulocyte- macrophage progenitor cells (GM-CFU-c), and increased the numbers of adherent macrophages. Addition of PMA to FLV-A-infected cultures did not inhibit generation of granulocytic leukemia cell lines even though the numbers of adherent adipocytes were decreased and adherent macrophages were increased. PMA treatment of freshly explanted whole bone marrow but not purified nonadherent GM-progenitor cells from long- term bone marrow cultures stimulated GM-CFU-c and cluster formation in the absence of added colony-stimulating factor (CSF). The sensitivity of purified GM-progenitor cells to L929 or WEHI-3 CSF was not altered by PMA; however, PMA treatment of bone marrow macrophages or peritoneal exudate macrophages stimulated detectable GM-CFU-c and cluster formation by purified GM-progenitor cells under conditions where equal numbers of untreated macrophages failed to be stimulatory. Thus, several PMA effects on hempoietic stem cells in vitro are mediated through indirect action on adherent stromal cells including macrophages.