We recently characterized a common form of genetic polymorphism in human apolipoprotein (apo) B, using the specific monoclonal antibody MB19 (Young SG, et al. Proc Natl Acad Sci USA 1986; 83:1101-1105). Antibody MB19 binds apo B with one of three distinct patterns of immunoreactivity (strong, intermediate, or weak). These three binding patterns are the result of the codominant inheritance of two common apo B alleles which encode for apo B allotypes MB19(1) and MB19(2), which have high and low affinity, respectively, for antibody MB19. Thus, subjects with strong or weak binding patterns are homozygous for MB19(1) or MB19(2), respectively, whereas those with an intermediate pattern are heterozygotes. To assess whether the MB19 polymorphism was related to hypercholesterolemia (HC) or to coronary artery disease (CAD), we determined the MB19 binding pattern and plasma lipoprotein concentrations in 129 normal subjects, 51 patients with HC, and 149 patients with CAD. The percentages of normal subjects having the strong, intermediate, and weak binding patterns were 11.6%, 41.9%, and 46.5%, respectively. The frequency of the three MB19 binding patterns was nearly the same in the groups with HC and CAD. Also, within each of the three groups of subjects, the MB19 binding pattern did not influence the plasma lipoprotein concentrations. We conclude that the genetic polymorphism in apo B defined by antibody MB19 is a common allelic variation in apo B, and that in the populations studied, it does not appear to be associated with the development of coronary artery disease or hypercholesterolemia.