The perivascular mononuclear cell collections of the rheumatoid synovium were examined both at the light and electron microscopic level by an immunoperoxidase staining technique using monoclonal antibodies directed against T cell subsets. These accumulations were variable in composition and size, not only in specimens from different patients but in the same specimen. Some areas (lymphocyte-rich areas) contained mainly small lymphocytes in clusters and others (transitional areas) contained blast cells, macrophages, and plasma cells in addition to lymphocytes. The percentage of T4 staining cells correlated positively and the percentage of T8 staining cells correlated negatively with the percentage of lymphocytes in any given area. In contrast, the percentage of T4 cells correlated negatively and the percentage of T8 cells correlated positively with the percentage of macrophage-like cells in these areas. Approximately 80% of the total lymphocytes, both in the lymphocyte-rich areas and transitional areas, were T lymphocytes (OKT3 staining). In lymphocyte-rich areas, helper/inducer T lymphocytes (OKT4 staining) were predominent over suppressor/cytotoxic lymphocytes (OKT8 staining), and in such areas the mean T4:T8 ratio was 2.9. Macrophage-like cells were seen only in small numbers in this type of area. In the transitional areas, suppressor/cytotoxic lymphocytes (OKT8 staining) predominated over helper/inducer lymphocytes (OKT4 staining). In such areas the mean T4:T8 ratio was 0.8. The T8 cells in the transitional areas tended to be large in size and often had a blastic appearance, and the abundant macrophage-like cells infiltrating these areas were frequently in close contact with T8 lymphocytes. These findings indicate that the ratio of T4 to T8 lymphocytes in rheumatoid synovium varies with the type of area examined. In lymphocyte-rich collections, made up largely of quiescent small lymphocytes, T4 cells are predominant. In areas of apparent immunological reactivity, T8 cells are predominant. It is suggested that T8 cells proliferate in immunologically active areas of the synovium as a result of local stimulation of a T cell-mediated immune response.