The loss in feedback control of cholesterol biosynthesis in tumor cells was examined in tissue culture. Human fibroblasts from normal subjects, SV40 tumor virus-transformed cell lines, and homozygous familial hypercholesterolemic cells as reference, were grown in tissue culture. Experiments were conducted to relate the regulatory enzyme for cholesterol biosynthesis, HMG CoA reductase, and the membrane-located binding receptors for low density lipoproteins (LDL) that mediate feedback control in normal cells. Monolayers of virus-transformed tumor cells exhibited specific (125)I-labeled LDL binding of 152 +/- 21 ng/mg cell protein, which was essentially the same as that of normal fibroblasts (135 +/- 20 ng/mg). Binding of LDL by familial hypercholesterolemic cells used as controls was only 8 +/- 3 ng/mg under the same test conditions. Basal levels of HMG CoA reductase in tumor cells of 45.2 +/- 6.5 units/mg cell protein were about twice those of normal cells. However, in contrast to the lack of feedback control of this enzyme observed with tumors in vivo, in both the normal and the transformed cells in vitro, activity of the enzyme decreased about fourfold when serum lipids were added. These findings demonstrate that tumor cells growing in vitro contain a normal complement of the membrane-located binding receptors for low density lipoproteins and, although the basal levels are higher than normal, an effective feedback regulation of the enzyme HMG CoA reductase is retained.