A monoclonal antibody against human apolipoprotein B (apoB),B₅,was used to test the immunnoreactivities of apo B in unrelated individuals. We found that the apo B in each person exhibited one of the three binding patterns: high,intermediate,or low affinity. Genetic investigation showed that these binnding patterns might result from co-dominant transmission of a pair of apo B alleles (B₅~+/B₅~-). The frequency of gene B₅~+ was 0.593 and B₅~-was 0.407. We also found that the frequency of B₅~+/B₅~+ homozygotes in the hypercholesterolemic patient group was significantly lower than that in healthy subject group. The serum levels of total cholesterol, triglycerides, very low density lipoprotein (VLDL)—cholesterol, and VLDL—triglyeerides in B₅~+/B₅~+ homozygotes were significantly lower than those in B₅~-/B₅~-in the group of healthy subject. It is suggested that allele B₅~+ might be associated with lower serum concentrations of lipids, especially the concentration of cholesterol.