Human B lymphocytes have been shown to have at least five polymorphic specificities defined by 32 antisera. The antisera were produced by absorption with pooled platelets to remove HLA activity and were selected out of over 400 tested sera. The sera that defined the five specificities had high correlation coefficients within a group (generally in the range of 0.6-0.9). As shown by the fit in the Hardy-Weinberg equilibrium, the five specificities appear to be determined by alleles at one genetic locus. No association between these specificities and HLA was noted.