BACKGROUND The production of natural autoantibodies incl. antinuclear antibodies (ANA) is ascribed to lymphocytes which have a CD5 molecule on their surface. The role of CD5 positive B lymphocytes in the induction of autoimmunity is obscure so far. The authors focused their attention on the incidence of antinuclear antibodies (AA) in subjects with different diseases and sought a relationship with the ratio of CD5 positive B lymphocytes in the peripheral blood stream. RESULTS CD5 positive lymphocytes were assessed on a flow cytometer using monoclonal anti CD5 and anti CD19 antibodies. Antinuclear antibodies are detected by indirect fluorescence on a substrate of human leucocytes and HEP-2 cells. In a group of 134 subjects the authors did not provide evidence of a direct relationship between the relative number of CD5 positive B lymphocytes in the peripheral blood stream and the presence of ANA (IgG, IgA, IgM), not even in 33 patients with autoimmune diseases. In 86 patients the authors revealed that antinuclear antibodies type IgM predominate in patients with repeated respiratory infections. In systemic diseases the isotype IgG predominates (p = 0.01). After immunosuppressive treatment with a favourable clinical effect the ANA isotype IgG disappears and isotype IgM is found more frequently. The incidence of the ANA isotype IgM is significantly higher in healthy subjects aged over 60 years than in younger subjects (12.5%/6.1%, p = 0.06), and more frequent in women (p = 0.05). CONCLUSIONS The presence of antinuclear antibodies is not associated with the amount of CD5 positive B lymphocyte in the peripheral blood stream.