To investigate mechanisms of anti-RNP antibody expression in autoimmune disease, idiotypes of a monoclonal anti-RNP of murine origin were analysed. This antibody, designated 4L1, was obtained from a MRL-lpr/lpr mouse and shown to have anti-RNP specificity by gel analysis of radiolabelled cellular RNA. An anti-idiotypic antiserum was prepared in a rabbit to 4L1 and rendered specific for idiotype by absorption with IgG from B6 mice and two BALB/c myelomas of the same chain composition as 4L1 (IgM kappa). In competition ELISA assays, this antiserum detected idiotypes commonly expressed in sera of MRL-lpr/lpr mice irrespective of the presence of anti-RNP. These idiotypes were not exclusive to this autoimmune strain, however, and could also be identified in normal mice. To identify other antibodies with this idiotype, a panel of MRL hybridomas was tested. This analysis demonstrated idiotypic cross-reactivity between 4L1 and two anti-Sm monoclonal antibodies derived from another animal. These results suggest that 4L1 belongs to a larger idiotype bearing family only some of whose members may have aberrant expression.