Cell-mediated immune responses of cultured synovial mononuclear cells against ureaplasmal, chlamydial, Mycoplasma hominis and mumps antigens were measured by the 3H-thymidine uptake method. Specific responses against ureaplasmal or chlamydial antigens were found in 3 of 4 patients with venereally-acquired Reiter's syndrome, but not in 3 patients with arthritis after episodes of diarrhea and 4 patients with rheumatoid arthritis.