We have developed a quantitative assay for IgG autoantibodies against IL-1 alpha using protein A-Sepharose CL-4B. We examined the autoantibodies in sera from 107 healthy subjects, 151 patients with rheumatoid arthritis (RA), 64 patients with systemic lupus erythematosus (SLE) and 16 patients with systemic sclerosis. The frequency of positive sera for the autoantibodies in patients with RA was 16.6%, which was about three times more frequent (P less than 0.01) than that in healthy subjects (5.6%) or that in patients with SLE (4.7%). Only one serum of 16 patients with systemic sclerosis was positive for the autoantibodies. Neutralizing activity of the autoantibodies was demonstrated by murine thymocyte proliferation assay. The concentrations of IgG at 50% inhibition of IL-1 alpha (15 pM) induced thymocyte proliferation ranged between 0.1 and 0.5 mg/ml. A time-course study showed fluctuations of the titres of the autoantibodies in parallel with the disease activity of RA. These results suggest that the anti-IL-1 alpha autoantibodies present in the sera and possibly some other body fluids may be involved in the regulation of IL-1 activity in vivo.