The effect of human IgG on human haemopoiesis has been studied in vitro. Dialysed purified IgG stimulated haemopoietic colony growth by bone marrow mononuclear cells (MNC) but not by monocyte-depleted MNC. Culture media, conditioned by IgG-stimulated peripheral blood MNC, augmented formation of neutrophil-macrophage, eosinophil, and megakaryocyte colonies by monocyte-depleted marrow MNC. Serum-free IgG-conditioned media also contained colony-stimulating activity (CSA). IgG Fc fragments and heat-aggregated IgG promoted the secretion of CSA, but F(ab')2 fragments, Fab fragments or ultracentrifuged IgG did not. In the cell-selection studies, CSA was produced by highly enriched monocytes following stimulation with Fc fragments. The antiserum against human granulocyte colony-stimulating factor (G-CSF) and/or granulocyte-macrophage CSF (GM-CSF) neutralized the CSA produced by Fc fragment-activated monocytes. Enzyme immunoassays showed G-CSF and GM-CSF in media conditioned by monocytes stimulated with the Fc fragments, heat-aggregated IgG and anti-D-sensitized red blood cells (RBC). Northern hybridization analysis showed mRNA encoding G-CSF and GM-CSF in RNA extracted from MNC and monocytes cultured with the Fc fragments, but not in the RNA from unstimulated cells or monocyte-depleted MNC. These results indicate that IgG Fc fragments, aggregated IgG and antigen-antibody complexes induce monocytes to produce G-CSF and GM-CSF in vitro. The CSFs release induced by IgG may be involved in the in vivo regulatory network in haemopoiesis.