A solid phase radioimmunoassay was developed for detecting the quantity of double-stranded and single-stranded DNA antibodies in patients with systemic lupus erythematosus and other connective tissue diseases. The assay system employs a solid support 96-well, flex-vinyl microtiter plate to which bovine methyl albumin is layered, followed by denatured or native calf thymus DNA. A 1:80 dilution of patients' sera was added to respective wells followed by tritiated high affinity anti-IgG, -IgA, or IgM. Denatured DNA (single-stranded DNA) bound to methylated bovine serum albumin had less than 5% reannealment to the double-stranded form and provided a better substrate for Ab binding than double-stranded DNA, producing a linear binding curve. Of 58 patients diagnosed as having systemic lupus erythematosus (SLE), only 11 having active SLE had IgG antibody levels of greater than 5.0 microgram/ml to single-strand DNA. Renal involvement of some degree was found in all 11 with the high concentrations of IgG antibodies to DNA correlating with severe involvement. Patients with IgM antibodies to DNA alone had more benign types of SLE with little renal involvement. No abnormal levels of IgA Ab to either single-strand DNA or double-strand DNA were found in SLE patients' sera. Corticosteroid and/or immunosuppressant treatment caused a marked drop in the IgM Ab level to DNA within 10 days while IgG Ab to DNA remained high for up to 30 days. Quantitation of IgG and IgM Ab to single-strand DNA provides a useful method for diagnosing severe SLE with possible renal involvement and monitoring the course of the disease during therapy.