Two randomised double blind, placebo controlled trials have been carried out to assess the effectiveness of nonathymulin, a synthetic thymic peptide hormone, in the treatment of rheumatoid arthritis (RA) and to compare three different dosage schedules (1, 5, and 10 mg/day). Nonathymulin 5 mg proved to be the most efficient dose, providing significant clinical improvement as evaluated by the global assessment of all patients who entered the trials (56% v 17% in the placebo group) (p less than 0.02) and by four objective parameters. This effect was accompanied with minimal adverse effects and was not associated with clear changes in immunological parameters. A significant correlation was observed, however, in clinical response to nonathymulin, and T cell subset imbalance was assessed using monoclonal anti-T cell antibodies and a functional suppressor T cell assay.