Immunochemical studies have demonstrated that apoprotein B-100 and apoprotein B-48 share some antigenic determinants, but whether they are products of the same gene has remained uncertain. Utilizing a specific mouse monoclonal antibody, MB19, we recently characterized a common form of genetic polymorphism that was expressed in apo-B-100 (Young, S. G., Bertics, S. J., Curtiss, L. K., Casal, D. C., and Witztum, J.L. (1985) Proc. Natl. Acad. Sci. U.S.A., in press). Antibody MB19 binds different allotypes of apo-B-100 (MB19(1) and MB19(2] with high and low affinities, respectively. Compared to a rabbit antiserum against human low density lipoprotein, which detects 100% of apo-B mass in all individuals, antibody MB19 detects 100% of apo-B with allotype MB19(1) but less than 10% of apo-B with allotype MB19(2). Western blots demonstrate that MB19 binds to both apo-B-100 and apo-B-48. To determine if apo-B-48 and apo-B-100 from the same individual express the same polymorphism, chylomicrons and very low density lipoproteins were isolated from 23 subjects in whom the allotypes of apo-B-100 were known. Delipidated apoproteins were separated electrophoretically and then transferred to nitrocellulose membranes. Nitrocellulose membranes were incubated with 125I-MB19 (to detect the polymorphism) and 131I-antiserum to human apo-B (to quantitate total apo-B transferred to nitrocellulose membranes). Apo-B-100 and apo-B-48 bands were removed and the ratio of 125/131 counts in each band was calculated. In all 23 subjects studied, the same MB19 polymorphism was present in both apo-B-100 and apo-B-48. This observation provides strong evidence that both apoproteins are products of the same gene.