41 patients with active rheumatoid arthritis entered a placebo-controlled double-blind randomised study in which 21 received slow intravenous injections (given in fractions over 10 min) of thymopentin (TP-5) 50 mg 3 times a week for 3 consecutive weeks and 20 received placebo in the same way. After 3 weeks of treatment the TP-5 group showed improvement (p less than 0.05 or p less than 0.01) in all but one of the clinical variables tested. There was improvement in the number of joints painful at rest, the number of joints painful on motion, scores for tenderness on pressure and swollen joints, severity of pain on awakening and morning stiffness, and right-hand grip strength; left-hand grip strength remained unchanged. In the placebo group, only morning stiffness improved significantly. The intergroup comparisons showed that thymopentin was significantly better than placebo in reducing tenderness, joint swelling, severity of pain on awakening, and disease activity. 4 weeks after the end of the TP-5 therapy, the improvement was still present although there was a trend towards relapses. No significant modifications occurred in any of the laboratory variables tested and only minor side-effects were experienced by either group.