Granulocyte-macrophage colony growth depends on the presence of colony- stimulating activity (CSA). Phorbol esters induce concentration- dependent colony formation in the absence of exogenous CSA. We questioned whether phorbol esters mimicked the action of CSA by directly stimulating colony growth, or whether phorbol esters acted indirectly by inducing marrow cells to release CSA. First, after incubating human bone marrow cells with phorbol 12,13-dibutyrate (PDB) for 3 days, we separated PDB from the protein peak of the conditioned medium by Sephadex G-10 gel filtration and tested this peak for the presence of CSA. When diluted 1:10 in the agar colony assay, this material induced 133 +/- 15 colonies/10(5) bone marrow cells. Second, to determine whether bone marrow cells required the continued presence of PDB in order to release CSA, PDB was removed from bone marrow cells by washing, and these cells were reincubated in fresh medium in the absence of PDB. CSA was found in the medium of these cultures; its release was maximal after preincubation of bone marrow cells with 5 X 10(-8) M PDB for 3 days, followed by incubation for 3 days in the absence of PDB. This CSA stimulated granulopoiesis out of proportion to monocytopoiesis, with 85% +/- 17% of the colonies being granulocytic (as indicated by histochemical staining for chloroacetate esterase), and 12% +/- 3% being monocytic (as indicated by nonspecific esterase). Inhibitors of monocyte colony formation, including PGE1, were not present in the medium that contained this CSA. These studies demonstrate that normal human bone marrow cells exposed to PDB release CSA and that this CSA selectively stimulates granulopoiesis in vitro.