Primary mouse peritoneal macrophages effectively take up and metabolize phosphatidylcholine/cholesterol liposomes containing a small amount of phosphatidylserine, which results in the massive accumulation in the cytoplasm of oil red 0 positive lipid droplets consisting of cholesteryl ester (CE) and triacylglycerol(TG)[Nishikawa et al.(1990) J. Biol. Chem. 265, 5226-5231]. A number of inhibitors of CE synthesis have been reported, but their effects on the lipid droplet formation have not been fully examined. Furthermore, the contribution of TG synthesis to lipid droplet formation has been poorly investigated. We have investigated the relationship between CE and TG syntheses and cytosolic lipid droplet formation in macrophages cultured in the presence of inhibitors with different modes of action. When macrophages were cultured with liposomes and [14C] oleic acid in the presence of triacsin C, a potent inhibitor of long chain acyl-CoA synthetase, both [14C] CE and [14C] TG syntheses were inhibited to similar extents with ICs, values of 0.19 and 0.10 ƒÊ M, respectively. On the other hand, pregnenolone, a well-known inhibitor of cellular cholesterol transport, and CL-283,546, a potent inhibitor of acyl-CoA: cholesterol acyltransferase, inhibited[14C] CE synthesis specifically with IC50, values of 5.0 and 0.038 ƒÊM, respectively. Microscopic observation revealed that the inhibitors of cholesterol metabo lism produce only a partial decrease in cytosolic lipid droplets even at the highest doses which cause almost complete inhibition of [14C] CE synthesis. However, the triacsin C-dose dependent inhibition of lipid droplet formation was almost complete at 0.59 ƒÊM, a concen tration that inhibits(14C] CE and[14C] TG syntheses by about 90%. These results show that inhibiton of acyl-CoA synthetase by triacsin C causes a decrease in the cellular levels of acyl-CoA, the common substrate for CE and TG syntheses, leading to an inhibiton of neutral lipid synthesis and eventually to the complete disappearance of cytosolic lipid droplets from macrophages. These findings suggest that TG synthesis, as well as CE synthesis, is responsible for macrophage-derived foam cell formation, and is therefore a potential target for new antiatherosclerotic agents.