A monoclonal antibody (BIP 45) against human apolipoprotein B (apo B) was used to study the polymorphism of apo B in families and in unrelated subjects. BIP 45 bound to apo B-containing lipoprotein particles in one of three distinct patterns of immunoreactivity (strong, weak and intermediate). Family studies showed that these binding patterns result from co-dominant transmission of apo B allelic pairs which are temporarily designated allele BIP- and allele BIP+; allele BIP+ would code for the apo B BIP 45 epitope. Analysis of plasma samples from 244 unrelated men randomly chosen from the North French population indicated that 46.7% of them bound BIP 45 with low affinity (weak reactors), 44.7% with intermediate affinity (intermediate reactors) and 8.6% with high affinity (strong reactors). According to the Hardy-Weinberg equilibrium, this corresponds to gene frequencies of 0.690/0.310 for the type BIP-/BIP+ alleles. This corresponds to the gene frequencies of 0.695/0.305 at the Ag(g)/Ag(c) locus previously found in a Caucasian population. Furthermore, the investigation of Ag(c,g) and of monoclonal BIP 45 antibody immunoaffinity for 30 individual plasma samples showed that BIP 45 bound strongly to Ag(c) factor, whereas it bound weakly to the allelic Ag(g) factor. This monoclonal antibody will be useful for the detection of the two corresponding apo B species designated apo B (Ag(c) factor, BIP+) and apo B (Ag(g) factor, BIP-).