The effect of procetofenic acid on 3-hydroxy-3-methylglutaryl Coenzyme A reductase (HMG-CoA reductase) activity has been studied on cultured human blood mononuclear cells. The drug produced a sharp decrease in enzyme activity both when it was added to the incubation medium and when it was added to the HMG-CoA reductase assay mixture. These findings strongly suggest that procetofenic acid inhibits HMG-CoA reductase activity by inactivating enzyme protein or by interfering with enzyme-substrate interaction. Since HMG-CoA reductase is a rate-limiting enzyme in cholesterol biosynthesis, its inhibition may explain, at least in part, the hypocholesterolemic effect of the drug.