138 patients, suffering from rheumatoid arthritis, were treated with sulfasalazine for up to 43 months. In 58.7% of the patients a good or moderate effect was ascertained. Side-effects occurred in 40.5% of the treated patients, mostly in the first 4 months after starting treatment, but some of the more serious adverse reactions occurred some months later. Presumably, drug-induced antinuclear antibodies were found in 29%, using Hep II cells and crithidia-luciliae flagellates. A lupus-like syndrome was not seen. Close controls of laboratory parameters are necessary including determination of liver enzyme levels a renal test, and blood counts according to the investigations in gold treatment. Detection of abnormal immunological reactions should be carefully scrutinized.