We have compared the NZB/B1NJ (/NJ) and NZB/B1Pt (/Pt) mouse substrains with respect to a number of immunological and biochemical polymorphisms. In agreement with earlier findings by other workers, we detected histocompatibility antigen difference(s) between the two substrains. Serological analysis indicated that /NJ and /Pt expressed the same allotypes of H-2, Thy-1 and Lyt-1 antigens, while they differed for the Lyt-2 antigen. Amongst 15 biochemical polymorphisms, which we assayed, /NJ and /Pt differed for four, namely Pep-3, Mup-1, Gpd-1 and Hbb. In addition, the two NZB substrains differed for the Hc marker (haemolytic complement).