The study of T lymphocyte subpopulations defined by monoclonal antibodies in a group of patients with rheumatoid arthritis and a control group indicated no significant differences in the percentages of total T lymphocytes (OKT3+) or helper-inducer lymphocytes (OKT4+). However there was a significant diminution in suppressor-cytoxic lymphocytes (OKT8+) in patients with active forms of the disease. This diminution led to an increase in the proportion of OKT4+/OKT8+ in patients with active R.A. forms compared to both those in remission and the control group.