Two monensin-resistant clones show similar low-density lipoprotein binding activity but less internalization or degradation of low-density lipoerotein than the parental Balb/3T3 or other resistant clone. Sterol synthesis from radioactive acetate in the resistant mutant, MO-5, is inhibited by more than 70% of control in the presence of tenfold higher amounts of low-density lipoprotein than the dose that inhibits the parental Balb/3T3 to similar level. 3-Hydroxy-3-methylglutaryl coenzyme A reductase activity of Balb/3T3 and MO-5 is inhibited by 48% and 27% of control, respectively, in the presence of 10 μg/ml of low-density lipoprotein. Colloidal silica gradient centrifugation shows that transport of low-density lipoprotein from the surface membrane to the lysosome is much slower in MO-5 cells than in Balb/3T3 cells. Down regulation of low-density lipoprotein receptors on the cell surface in Balb/3T3 is observed by exposing the cells to 5–15 μg/ml low-density lipoprotein, whereas only slight if any down regulation is observed when MO-5 cells are treated with low-density lipoprotein. The altered endocytosis of low-density lipoprotein behaves as a dominant trait in hybrids of MO-5 and THO2-2, a derivative of Balb/3T3 resistant to both ouabain and 6-thioguanine.