A monoclonal antibody (MB-19) was used to investigate the polymorphism of apolipoprotein B in a large East Finnish family and in unrelated subjects. Apolipoprotein B was shown to exhibit high, intermediate or low affinity binding to this antibody. Thus, MB-19 bound strongly to the Ag(c) epitope, an Ag antigenic domain previously characterized by human antisera, while it bound only weakly to the allelic epitope Ag(g). It proved useful for the detection of the two corresponding allelic apoB species designated apoBc (= high affinity binding) and apoBg (= low affinity binding), and for confirming their co-dominant transmission. Intermediate binding resulted from the presence of a mixture of both apoB populations in heterozygous subjects.