We have identified and characterised three new idiotypes on human IgM McAbs generated from the splenocytes of a SLE patient with active disease. RT-6, which binds H1 and Sm/RNP, expresses essentially a private Id. Its expression is limited to a small number of human McAbs and the sera from patients with infectious diseases. In contrast RT-72Id and RT-84Id, expressed on McAbs which are polyreactive for two or more antigens, have a public distribution. RT-72Id and RT-84Id are found on McAbs from murine and human adult, and foetal tissues. In sera, significant numbers of SLE, RA and patients with other autoimmune diseases are positive for both Ids. RT-84Id is also elevated in SLE relatives and spouses, and in patients with Klebsiella infection. No correlation with disease activity, IgM or IgG levels was observed with either Id. However, RT-72Id was significantly associated with anti-ssDNA antibodies and RhF. RT-6Id and RT-72Id are located on the framework regions of the mu heavy chain, whereas RT-84Id is present on the kappa light chain, within the binding site. The McAbs are encoded by mainly germline genes: heavy chains of RT-6, RT-72 and RT-84 are encoded by the genes VH26, VH4.22 and VH4.21, respectively, and the light chain sequences of RT-6 and RT-72 are derived from DPL11 and HK102. Immunofluorescent staining revealed the presence of RT-72Id and RT-84Id positive immunoglobulin deposits in 18% and 45%, respectively, of the lupus renal sections compared with none in the disease control group, suggesting that these Ids may contribute to the pathology of the disease.