Four hybridomas are described which produce antibodies to the RI-1b rat kappa chain allotype. Three are rat alloantibodies of KG2b isotype, one is a mouse heteroantibody of KG2a isotype. These antibodies were labeled by incorporation of 75Se-selenomethionine and their specificity determined by a plate-binding assay. Inhibition studies using partially cross-reactive sera from Asian and Australian Rattus show that these four hybridomas detect four distinct RI-1b specificities. Differences in the slopes of the inhibition curves indicate that each of the specificities may exist in more than one form. In addition, in the form of 125I-labeled antibodies, two of these hybridomas show a high degree of specificity in binding to Ig-bearing lymphocytes from spleen and lymph nodes. These hybridomas will therefore be useful not only in studies on the phylogenetic distribution of RI-1 specificities but also in replacing conventional antisera for immunochemical and cellular immunological studies.