Fifty-seven serum samples positive for antibodies to double-stranded DNA (dsDNA), single-stranded DNA (ssDNA) or small nuclear ribonucleoproteins (snRNP) selected from patients with systemic lupus erythematosus and mixed connective tissue disease, were examined for the IgG subclass distribution of these autoantibodies. It was shown that antibodies to dsDNA were relatively restricted to IgG1 and IgG3 subclasses whilst antibodies to ssDNA were equally distributed throughout the four subclasses. Antibodies to snRNP were essentially restricted to the IgG2 isotype. These isotype distribution patterns contrasted with that observed for total serum IgG.