Thirty patients with active rheumatoid arthritis (RA) participated in an open study of 6 months' treatment with either 5-aminosalicylic acid (5-ASA) or sulphapyridine (SP), the two moieties of sulphasalazine (SASP). Patients were assessed at regular intervals using clinical and biochemical tests designed to detect specific antirheumatic activity. Patients taking SP showed significant improvement in disease activity, but those taking 5-ASA did not improve, despite the fact that high serum concentrations of 5-ASA and acetyl 5-ASA were achieved. These results suggest that SP is the active moiety of SASP. Its possible mode of action is discussed. Nausea was a frequent problem in patients taking SP. Unless this can be overcome, SP is unlikely to offer any therapeutic advantages over SASP in the treatment of RA.