Using a stepwise selection procedure, we have developed a line of mouse FM3A cells which is resistant to compactin (ML-236B), a competitive inhibitor of 3-hydroxy-3-methyl-glutaryl-coenzyme-A (HMG-CoA) reductase. This line, designated CR200, expressed a stable phenotype of a greater than 100-fold higher resistance to the inhibitor and a 160-fold higher HMG-CoA reductase activity than did the parental FM3A cell line when grown in the presence of low-density lipoprotein (LDL). In contrast to its action in the parental cells, HMG-CoA reductase in CR200 cells was not suppressed by human LDL. The activity of both cell-surface binding and degradation of 125I-LDL was approximately 10% in CR200 cells as compared with that in parental cells. Affinity of the 125I-LDL binding in CR200 cells was similar to that in the parental cells. In addition, HMG-CoA reductase was not suppressed normally by mevalonate and 25-hydroxycholesterol in CR200 cells; suppression was 50-60% by both compounds under conditions where suppression was about 99% in the parental cells. Sterol synthesis from [3H]mevalonate was normal in the variant cells. These observations demonstrate two defects in CR200 cells: decreased levels of LDL receptor and a defective regulation of HMG-CoA reductase by one or more products derived from mevalonate. When grown without compactin for 21 weeks, the variant cells became slightly sensitive to compactin and concomitantly recovered partly from both defects. However, these phenotypes of CR200 cells were stable up to 15 months when exposed to compactin. These two defects may account for the overaccumulation of HMG-CoA reductase in CR200 cells and thereby for the resistance to compactin.