The perivascular infiltrates of the rheumatoid synovium were examined in the electron microscope. Lymphocyte-rich, plasma cell-rich, and transitional areas were observed. The transitional areas contained lymphocytes, plasma cells, blast cells, macrophages, and fribroblasts. Although lymphoblasts were frequent, the blast cells were predominantly plasmablasts. Marked degeneration of fibroblasts in the vicinity of lymphoblasts suggested the liberation of a lymphotoxin by the lymphoblasts. There was close contact between blast cells and macrophages and between macrophages and lymphocytes. The close association of lymphoblasts, plasmablasts, and macrophages in the transitional areas suggests that these are sites of T- and B-cell interaction in the rheumatoid synovial immune response.