Stimulated peripheral blood mononuclear cells (MNC) are one of the richest described physiologic sources of colony-stimulating activity. To understand the molecular basis for, and the cellular sources of, this MNC activity, we cultured purified human lymphocytes and monocytes for 2 hr to 6 days and examined colony-stimulating factor (CSF) gene activity by Northern blot analysis. We show that MNC are capable of expressing messenger RNA for macrophage (M)-CSF, granulocyte (G)-CSF, GM-CSF, and multi-CSF when stimulated with mitogens. The time courses of induction of these genes differ, with G-CSF induction preceding that of the other CSFs. In addition, the spectra of CSFs produced by cell populations enriched for lymphocytes, monocytes, or macrophages differ. The implications of these findings for the selective activation of hematopoiesis are discussed.