Peritoneal exudate cells were collected from thioglycollate stimulated mice, extracted an examined for the presence of immunoreactive and bioactive fibroblast growth factor (FGF). The crude extract stimulated in a dose dependent fashion the proliferation of vascular endothelial cells derived from the bovine aortic arch. The extract also showed a parallel and dose-dependent inhibition of binding in a highly specific radioimmunoassay for FGF. The immunoreactive FGF (ir-FGF) contained in the extract was retained on a heparin-sepharose affinity column as is characteristic of pituitary FGF. Reverse-phase HPLC of the macrophage-derived material reveals one biologically active form of FGF which coelutes with the major form of immunoreactivity. The results demonstrate the presence of FGF in these cells and suggest that at least one of the hitherto unidentified mitotic activities in these extracts is due to a mitogen indistinguishable from FGF.