The lymphocyte responses in autologous mixed lymphocyte reactions (AMLR) between irradiated non-T and T lymphocytes from the peripheral blood (PB) of rheumatoid arthritis (RA) and juvenile RA (JRA) patients were decreased compared with the AMLR responses of normal PB lymphocytes. Normal AMLR responses were seen in synovial tissue and the synovial fluid lymphocytes from RA and JRA patients. The lymphocyte responses were also decreased in AMLR between irradiated non-T cells from peripheral blood and T cells from synovial tissue (ST) in RA patients and between irradiated non-T from PB and synovial fluid (SF) T cells in JRA patients. However, when irradiated non-T cells from ST of RA patients or from SF of JRA patients were mixed with autologous PB T lymphocytes, increased lymphocyte responses were observed. SF T lymphocytes and ST T cells were also shown to stimulate autologous PB T lymphocytes.