It was found that human serum stored for 2 months at 4°C (modified serum) induced monocyte proliferation and simultaneous macrophage colony stimulating factor (M‐CSF) production by these cells in vitro. Cell number, estimated by DNA content, doubled after 10 days in culture in the presence of modified serum, while it decreased in culture with freshly thawed control serum. As the addition of more than 2.5 ng/ml of recombinant M‐CSF significantly supported monocyte survival/proliferation, cells were cultured for 10 days in medium supplemented with control serum, and endogenous M‐CSF production was investigated by enzyme‐linked immunosorbent assay. M‐CSF concentration in the supernatants was 15–30 ng/ml after 10 day in culture with modified serum, a level that might be sufficient for monocyte proliferation. The modified serum induced M‐CSF from freshly isolated monocytes, while M‐CSF was hardly detected in cultures supplemented with control serum. Assay for peroxidized lipid and agarose gel electrophoresis demonstrated that the modified serum contained more oxidized low density lipoproteins (LDL) than the control serum. Ligands of scavenger receptors, which are receptors for oxidized LDL, such as dextran sulphate, polyinosinic acid, heparin and acetylated LDL also significantly induced M‐CSF production from human monocytes, although this was at levels below 2 ng/ml. These results indicate that serum modified by oxidation stimulates monocytes to produce M‐CSF resulting in their proliferation, and that signalling via scavenger receptors is one of the mechanisms responsible for this induction of M‐CSF.