Recently, we have used synthetic peptides corresponding to the complementarity-determining regions (CDR) of Ig molecules to induce antiidiotypic antisera. Peptide PSH3, representing the third CDR of the IgM rheumatoid factor (RF) Sie heavy (H) chain, induced a private antiidiotype that reacted with only one out of five IgM-RF. Peptide PSL2, corresponding to the second CDR of Sie light (L) chain, induced an antibody against a crossreactive idiotype (CRI), expressed by 10 out of 12 human IgM-RF analyzed. Herein, we report that five additional antiidiotypic antibodies were generated by immunization with synthetic peptides identical to the third L chain CDR of IgM-RF Sie (PSL3), the second and third H chain CDR of IgM-RF Wol, and the second and third CDR of IgM-RF Pom. As analyzed by immunoblot assay, both anti-PSL3 and anti-PSL2 reacted with the majority of 16 IgM-RF. In contrast, all five antiidiotypes induced by the H chain peptides reacted only with the parent proteins, except anti-PSH3, which reacted weakly with one additional RF. These results suggest that one (or very few) VL gene(s), but a larger number of VH genes, are used to encode IgM-RF autoantibodies.