Radiosensitive T cell suppression was studied in relation to the level of T cells with IgG-Fc receptor (TG cells) in synovial fluid and blood of nine patients with rheumatic diseases. Irradiation of T cells significantly decreased the proportion of TG cells in the blood of patients and healthy controls. The proportion of synovial TG cells was low in all patients before as well as after irradiation. Irradiation enhanced the helper effect of T cells on pokeweed-mitogen induced immunoglobulin production by allogeneic B cells. This increased helper effect was taken as an expression of decreased suppressor activity following irradiation. By this method suppressor activity was demonstrable in synovial T cells of six out of nine patients and in blood T cells of six out of eight patients and six out of eight healthy subjects. Thus the low level of synovial TG cells was in most patients not associated with a lack of radiosensitive synovial suppressor cell activity.