The responses of synovial lymphocytes to Chlamydia/Ureaplasma and to enteric antigens were studied in 31 patients with arthritis confined to knee joints, 15 patients with sexually-transmitted Reiter's syndrome, 9 with enteric Reiter's syndrome, and 24 with rheumatoid arthritis. The frequency of HLA antigens was studied in 28 patients with knee joint arthritis; this group was characterized by elevated frequencies of HLA-A2 and DR1. A subgroup of 8 responders to Chlamydia/Ureaplasma was characterized by an increase of HLA-Bw44 and DR7 or 8, while a subgroup of 8 responders to enteric antigens was characterized by increases of HLA-A1 and DR5. The frequency of HLA-B27 in the groups responding to antigens was 25-30%, less than half the frequency in patients with Reiter's syndrome.