Incubation of mutant Niemann‐Pick C fibroblasts with low‐density lipoprotein (LDL) resulted in excessive internalization of lipoprotein and extensive cellular over‐accumulation of unesterified cholesterol. The uptake of LDL by the mutant cells appeared to occur through the classic LDL receptor pathway and internalized lipoprotein was processed in lysosomes. Lipoprotein uptake into mutant cells was associated with delays in the initiation of established cellular cholesterol homeostatic responses. Subcellular fractionation of mutant Niemann‐Pick C fibroblasts accumulating LDL‐cholesterol showed excess unesterified sterol to be localized in the light lysosome‐light membrane region of a Percoll gradient, and revealed that cholesterol storage was associated with a specific alteration in the normal profiles of lysosomal marker enzymes.— Pentchev, P. G.; Comly, M. E.; Kruth, H. S.; Tokoro, T; Butler, J.; Sokol, J.; Filling‐Katz, M.; Quirk, J. M.; Marshall, D. C.; Patel, S.; Vanier, M. T.; Brady, R. O. Group C Nieman‐Pick disease: faulty regulation of low‐density lipoprotein uptake and cholesterol storage in cultured fibroblasts.FASEB J.1: 40‐45; 1987.