The accumulation of cholesteryl ester in mouse peritoneal macrophages and in bovine aortic smooth muscle cells by adding the mildly modified LDL (mm-LDL) was greatly enhancedwhen these cells were preincubated with monocyte colony-stimulating factor (M-CSF),indicating the synergism of mm-LDL and M-CSF on the lipid accumulation in cells. We also found that M-CSF increased the oxidative capability of cultured macrophages. This might be one of the mechanisms of synergism of mm-LDL and M-CSF.