Six indices of low density lipoprotein (LDL) receptor activity were assayed in cultured fibroblasts from seven subjects with familial hypercholesterolemia (HC) and six subjects without HC (non-HCs). Four non-HCs, three HC heterozygotes and one HC homozygous proband belonged to one kindred (kindred A). The proband's fibroblast 125I-LDL processing values fell within or were slightly above the range defined by fibroblasts from three "receptor-negative" HC homozygotes. Thus, the plasma membrane receptor defect in this kindred is probably of the "receptor-negative" category. LDL receptor-dependent 125I-LDL processing was about twice as high in fibroblasts from non-HCs as in those from HC heterozygotes belonging to kindred A. The segregation pattern of LDL receptor activity in this kindred was compatible with control by a single gene locus. 125I-LDL processing values from non-HCs, HC heterozygotes and HC homozygotes differed significantly from one another, but non-HCs and HC heterozygotes showed some overlap. LDL receptor-dependent 125I-LDL association (plasma membrane binding plus intracellular accumulation) data for 6 HC heterozygous and 13 non-HC fibroblast strains clustered into two and into three groups, respectively. Median 125I-LDL association levels in these groups appeared to be in agreement with hypothesis that two different geno-types in HC heterozygotes and three in non-HCs determined LDL receptor activity. These findings suggest the possibility that 125I-LDL processing studies may reveal "normal" alleles at the LDL receptor locus.