3H-thymidine uptake responses by synovial mononuclear cells to stimulation with sexually transmitted, enteric and mumps antigens were studied in 12 patients with "sexually transmitted Reiter's syndrome", 5 with "enteric Reiter's syndrome", 5 with rheumatoid arthritis, 4 with ankylosing spondylitis and 10 with "indolent arthritis of one knee." The "sexually transmitted" and salmonella cases were distinguishable by the responses. Synovial responses were sometimes marked when peripheral blood responses were negligible.