The etiology of rheumatoid arthritis is unknown. Virus infection is one of alleged factors initiating the disease process. In view of this, the authors undertook a trial of administration of an antiviral and immunostimulating preparation - isoprinosine, given in the early phase of the disease prior to the development of erosions. Isoprinosine was given to ten seropositive cases of rheumatoid arthritis (two males, eight females) aged 34-61 years, with disease duration from 6 months to 5 years treated previously only with non-steroidal anti-inflammatory drugs. Isoprinosine was administered for 4 weeks (2 weeks of 3.0 g daily and 2 weeks of 1.5 g daily). After the treatment, a statistically significant reduction was observed in the number of joints painful on pressure, the number of swollen joints, and the duration of morning stiffness. The grip strength of the left hand was increased. The ESR and serum uric acid level were not changed significantly. The titre of Waaler-Rose reaction in four patients was decreased, in two it was increased, and in four it remained unchanged. No side effects were observed during the treatment. In the light of these observations it seems worthwhile to continue further studies on the action of isoprinosine in early rheumatoid arthritis.