The mixed cryoglobulinemia is considered to be a nonmalignant human B-cell proliferation that frequently produces a monoclonal IgM with anti-IgG activity (rheumatoid factor). Using murine monoclonal anti-idiotypic antibodies specific for private or minor idiotopes on monoclonal IgM from three patients suffering from nonmalignant mixed cryoglobulinemia, we investigated the presence of the CD5 antigen on the monoclonal IgM producing cells in these patients. It is shown by two-color cytofluorometric analysis that the majority of the peripheral blood monoclonal IgM rheumatoid factor secreting cells is CD5 negative in these three patients. One of the monoclonal rheumatoid factor K variable regions was sequenced at the protein level and belongs to the human VK III group, as a high proportion of monoclonal rheumatoid factors and some B-cell chronic lymphocytic leukemia (CLL) membrane bound Igs. Thus, despite the preferential use of similar VK genes and the absence of somatic mutation affecting these variable regions in both malignant B-cell CLL and nonmalignant mixed cryoglobulinemia, these proliferating B cells differ in the CD5 membrane expression.