Familial defective apolipoprotein B-100 (FDB) is a genetic disorder presenting with hypercholesterolemia 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 gene. In the present study thirteen members of a family with moderate hypercholesterolemia (250-350 mg/dl) were investigated. Biochemical studies on cultured skin fibroblasts ruled out classical familial hypercholesterolemia (receptor deficiency). LDL from nine affected members displayed, in an "in vitro" cell binding assay, a reduced affinity (2.5 fold) for the receptor, and had normal electrophoretic mobility, size and chemical composition. Lp(a) levels in family members were comparable to those present in normolipidemics and lower than those observed in primary hypercholesterolemia. The disorder is transmitted over three generations as an autosomal codominant trait and all the affected members are heterozygotes and hypercholesterolemic.