Familial defective apolipoprotein (apo) B-100 is a recently described genetic disorder that appears to result from a mutation in the apoB-100 gene. This disorder is characterized by hypercholesterolemia resulting from elevated plasma concentrations of low density lipoprotein LDL. The disorder was first detected in three members of one family. The LDL from affected subjects binds defectively (approximately 30% of normal) to LDL receptors, retarding the clearance of LDL from plasma. In the present study, two other members of the affected family were found to possess abnormal LDL. In addition, abnormal LDL with a similar binding defect were found in a second, unrelated family. In both families, the defect is transmitted over three generations as an autosomal codominant trait and all affected members are heterozygotes. Since there is only one apoB-100 molecule per LDL particle, the abnormal LDL in heterozygous subjects is made up of two populations of particles: one that has normal binding activity to receptors and one that binds defectively. To localize the mutation in apoB-100, the binding of five apoB-100-specific monoclonal antibodies to abnormal LDL was assessed in a solid-phase RIA. Only antibody MB47, whose epitope is between residues 3350 and 3506, distinguished abnormal LDL from normal LDL isolated from control subjects with normal lipid levels; MB47 bound with a higher affinity (by approximately 60%) to abnormal LDL. In every individual with abnormal LDL, the MB47 antibody bound with a higher affinity. The convenience of this assay will facilitate screening of large populations to determine the frequency of this disorder.