Recombinant human alpha-interferon is now under intensive investigation as therapy for chronic Type B hepatitis. Recent reports have suggested that prolonged alpha-interferon therapy may induce autoimmune reactions. We have evaluated the problem of autoimmunity related to alpha-interferon therapy by testing for 15 different antibodies in the sera of 31 patients treated with alpha-interferon. No patient had autoantibodies before treatment; 27 (87%) of 31 patients developed at least one autoantibody. Eleven patients had antinuclear antibodies and 21 had smooth muscle antibodies, both of which usually developed during alpha-interferon therapy. In contrast, antibodies to endocrine organs such as thyroid microsomal, thyroglobulin and parietal cell antibodies arose in 12 patients, but usually several months after alpha-interferon treatment. The appearance of these autoantibodies did not correlate with disease activity or response to alpha-interferon. No patient developed autoantibodies specifically associated with autoimmune liver diseases such as liver kidney microsomal antibodies, autoantibodies to soluble liver antigen and the primary biliary cirrhosis specific subtypes of antimitochondrial antibodies. These results suggest that prolonged alpha-interferon therapy can induce autoantibody production and, in susceptible patients, may lead to autoimmune disorders.