The addition of mixtures of anti-allotypic sera to cultures of lymphocytes obtained from allotypically heterozygous donors causes a greater amount of “blast” transformation than the sum of the maxima that can be obtained with each anti-allotypic serum added separately. This “superaddition” of anti-allotypically induced transformation implies that the expression of allotypic specificities by lymphocytes is not limited to one allele (allelic exclusion), as has been found for plasma cells. Therefore, the expression of genetically controlled immunoglobulin antigenic specificities by lymphocytes appears to be less restricted than that by plasma cells. Plasma cells may become restricted in regard to immunoglobulin expression during differentiation from precursor cells that are not so restricted.