Familial defective apolipoprotein (apo) B-100 is a genetic disorder presenting with hypercholesterolaemia and abnormal low-density lipoprotein (LDL) that binds poorly to LDL receptors. This disease appears to be caused by a mutation in the apo B-100 gene. In the present study thirteen members of a family with moderate hypercholesterolaemia (250-350 mg dl-1) were investigated. Biochemical studies on cultured skin fibroblasts ruled out classical familial hypercholesterolaemia (FH, receptor deficiency). We then studied the interaction between LDL and their receptors by an in vitro cell binding assay. LDL from nine affected members displayed a reduced affinity (2.5-fold) for the receptor, and were less effective than LDL from control and unaffected members in suppressing LDL receptor expression and in stimulating cholesterol esterification. LDL from the affected members had normal electrophoretic mobility, size and chemical composition. Partial delipidation did not modify the LDL binding defect. The disorder is transmitted over three generations as an autosomal codominant trait and all the affected members are heterozygotes and hypercholesterolaemics. Analysis of DNA from family members showed a point mutation leading to an Arg to Gln substitution at amino acid 3500 of the mature protein that segregated with hypercholesterolaemia and LDL defective binding. We conclude that this family is affected by familial defective apolipoprotein B-100 (FDB).