AbstractTwo allotypic specificities (W‐1 and SD‐1) of rat immunoglobulin are described. These allotypic specificities are detected by a passive hemagglutination assay. Both specificities are located on the L‐chains of rat immunoglobulin and were found to be present in both IgG2and IgM. The gene loci controlling the allotypic determinants are inherited in a simple Mendelian fashion. Rats of 27 different strains tested possessed either the W‐1 or the SD‐1 sp