In this paper we report data regarding the IgM Y7 cross-reactive idiotope (CRIo) obtained by analysis of: 1) its V-gene subgroup dependance, 2) the frequency of its expression on human monoclonal IgMs and IgM molecules from normal and pathological sera. Furthermore, comparison of epitopic repertoire and nature of binding of human monoclonal IgMs expressing Y7 CRIo was performed to confirm the natural antibody properties of these molecules. IgM isolated from sera of patient DJ (IgM DJ) which expresses the Y7 idiotope has been classified to VH3/VL2 subgroup. From ten IgMs tested only IgM from patient RD (IgM RD) has been shown to express Y7 idiotope. Y7+ human IgMs bound to ssDNA, lactic acid bacteria, mouse laminin, porcine thyroglobulin and mouse IgG. Higher percentage of the expression of Y7 CRIo was detected in the sera of patients suffering from autoimmune diseases such as lupus, rheumatoid arthritis and psoriasis vulgaris as well as in patients suffering from chronic infections of the lower urinary tract. Antigen binding repertoire and properties of Y7+ monoclonal IgM, frequency of Y7 expression on monoclonal IgMs and its concentration in normal and pathological sera indicate the important biological role of this CRIo within the immune system.