Synovial tissues from 5 patients with rheumatoid arthritis (RA) were examined with immunofluorescence microscopy for the presence of lymphocytes with either bone marrow-derived (B) or thymus-derived (T) surface markers. Five synovial tissues with severe to mild lymphocytic infiltrations by bright field microscopy were examined in parallel with immunofluorescence. B cells were identified with a pepsin-digested fluoresceinated anti-F (ab'), antiserum and T cells were detected with a specific rabbit anti-T lymphocyte antiserum. By these techniques 75-902 of the lymphocytes in these frozen sections were identified as T cells. Cell suspensions were also prepared by collagenase digestion of two of the five synovial tissues. The lymphocytes in these cell suspensions were predominantly T lymphocytes (7885%) as shown by their ability to form spontaneous rosettes with sheep erythrocytes (E rosettes).