The natural killer cell activity of synovial fluid mononuclear cells and synovial membrane mononuclear cells from patients with rheumatoid arthritis was compared with that of paired peripheral blood mononuclear cells from many of these patients. Natural killer cell activity was measured by the use of a conjugate-binding cytotoxicity assay with an erythroleukemic cell line, K562, as target. There was no significant difference when 15-minute target binding by peripheral blood mononuclear cells was compared with synovial fluid mononuclear cells. Target binding of synovial greater (P less than 0.05) than that of peripheral blood cells. Three-hour target killing, however, was significantly greater when synovial fluid mononuclear cells were compared with the peripheral blood cells, P less than 0.01, and when synovial membrane cells were compared with the peripheral blood cells (P less than 0.05). The products of binding and 3-hour killing, a reflection of the total number of mononuclear cells participating in cytotoxicity, were significantly greater when either synovial fluid or synovial membrane mononuclear cells were compared with peripheral blood mononuclear cells (both P less than 0.01). Electron microscopy confirmed the presence of large granular lymphocytes, representing 20% of the peripheral blood cells and 37% of the synovial fluid mononuclear cells. Interferons were detected in 10/12 rheumatoid and 3/12 nonrheumatoid synovial fluid samples studied. These findings indicate that functional natural killer cells are selectively increased in the rheumatoid joint and may contribute to the overall increase in immunologic activity found in the joints of these patients.