A monoclonal antibody directed against human apolipoprotein B, which was previously shown in family studies to detect allelic variations (J Biol Chem 1984; 259:6423-6430), has now been identified with the Ag(c) factor. This identification allows the location of the Ag system on the structural gene for apolipoprotein B and on the short arm of human chromosome 2. The epitope corresponding to Ag(c) is located within the amino acid sequence common to apolipoproteins B-100 and B-48. Since a single molecule of apolipoprotein B-100 is present on human LDL, individual LDL possesses either the epitope corresponding to Ag(c) or that corresponding to Ag(g). These studies on allelic variation among human apolipoprotein B species parallel similar studies in animals in which a relationship to atherosclerosis was found.