A soluble activity inhibiting over 50% of the CTLL-2 cell line response to recombinant human interleukin 2 (IL 2) was found in 17 of 29 (59%) rheumatoid synovial fluids. To study the prognosis value of this activity, 16 rheumatoid synovial fluids were collected before a radiation synovectomy of the knee with 7 mCi of 90Yt. Patients with a good clinical result after the synovectomy had a lower IL 2 inhibitory activity than those with a bad or incomplete result (P less than 0.01). Levels of inhibitory activity and of soluble IL 2 receptors were correlated with each other and with the response of the synovitis to the radiation synovectomy. These results extend the clinical usefulness of soluble IL 2 receptor measurements and indicate a correlation between the immune activation of the rheumatoid synovitis and its clinical activity.