To investigate the mechanisms of anti-Sm expression in murine systemic lupus erythematosus (SLE), the idiotypic determinants of a monoclonal anti-Sm antibody were studied. This antibody, 2G7, was derived from the fusion of spleen cells of an autoimmune MRL-lpr/lpr mouse with the 653 myeloma. Specificity for Sm was demonstrated by an ELISA with the use of affinity-purified Sm as well as immunoprecipitation of radiolabeled RNA. An anti-2G7 antiserum was prepared in a rabbit and rendered specific for idiotype by extensive absorption against BALB/c and MRL-lpr/lpr monoclonal proteins. The resulting antiserum detected determinants found on 2G7 as well as on two MRL anti-DNA antibodies, 6/P and 6/N, an independently derived MRL-lpr/lpr anti-Sm called 7.13, and the BALB/c myeloma FLOPC 21. Two distinct determinants could be demonstrated by the creation of cross-reactive idiotype systems by using the various monoclonal antibodies as ligands for the anti-idiotype. Both idiotypes were demonstrated in sera of normal and autoimmune mice, although MRL +/-/+/- mice had the highest levels of strains tested. To explain the pattern of idiotypic relatedness, 2G7 was tested for anti-DNA activity by ELISA. 2G7 displayed activity for single-stranded DNA as well as synthetic DNA and RNA homopolymers. Absorption analysis indicated that the anti-DNA and anti-Sm binding activities were the product of the same antibody. These results suggest that anti-Sm and anti-DNA may be related by both idiotype and antigenic specificity, providing a mechanism for their common expression in SLE.