The suspended growth of LM cells in a lipid-free chemically defined medium was almost completely inhibited in the presence of 0.1 μg/ml of ML-236B, a potent competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate limiting enzyme in cholesterol biosynthesis in mammalian cells. This inhibition was effectively counteracted by adding a small amount of either mevalonate or cholesterol (dispersed in delipidated calf serum) to culture medium. The synthesis of desmosterol, the end product of sterol biosynthesis in LM cells, from [¹⁴C] acetate in cultured cells was highly sensitive to ML-236B, being inhibited 35 and 60% at its concentrations of 0.1 and 1 ng/ml, respectively, while the incorporation of [³H] mevalonate into desmosterol was not affected by ML-236B at concentrations up to 0.1 μg/ml. Synthesis of fatty acids, phospholipids, triglycerides and macromolecules like DNA, RNA and protein were not suppressed by 10 μg/ml of ML-236B. Desmosterol content of LM cells was reduced by treatment with ML-236B. These results indicate that ML-236B inhibited cell growth via specific interference in the pathway of sterol biosynthesis, presumably on the step catalyzed by HMG-CoA reductase.