Cells of the macrophage lineage are a major source of various cytokines and hematopoietic growth factors. With regard to the growth factors acting on cells of their own lineage, macrophage colony-stimulating factor (M-CSF) has been proven to be secreted by monocytes (MO) and macrophages (MAC), whereas the production of granulocyte-macrophage colony-stimulating factor (GM-CSF) by human MO/MAC is under debate. Here we report that in elutriation-purified MO, as well as in MAC derived from cultured MO, GM-CSF m-RNA was regularly induced by LPS. In MO the GM-CSF message was still detectable 18 h after stimulation under serum-free conditions, but in contrast was already lost at this time point in MAC. Secreted GM-CSF protein was detected in the culture medium using a sandwich ELISA. Furthermore, a factor-dependent cell line (M-07) was used for a biological assay. Here, a neutralizing anti GM-CSF antibody specifically blocked the proliferation-inducing activity of MO/MAC supernatants. Whereas only small amounts of GM-CSF were detected in MO, its secretion increased severalfold upon MO-to-MAC differentiation in vitro. A similar increase upon in vitro maturation of MO was observed for the production of granulocyte colony-stimulating factor. The highest amounts of GM-CSF (up to 2.8 ng/10(6) cells) were produced by MAC that had been derived from MO cultured under serum-free conditions in the presence of 0.5 mg/ml albumin as the only medium supplement.