Lymphocytes from the synovial fluid of patients with rheumatoid arthritis were examined for the expression of granzyme A and perforin. Previous studies have demonstrated that the expression of these proteins, which are implicated as mediators of cytotoxicity, can be used to identify putative cytolytic lymphocytes in vivo. Twenty-two synovial fluid samples were analyzed by in situ hybridization and immunohistochemistry. In six patients receiving low doses of immunosuppressant, a population of granzyme A- and perforin-expressing lymphocytes could be identified. In contrast, lymphocytes from patients who were receiving high doses of immunosuppressant did not contain any granzyme A- or perforin-expressing lymphocytes. Synovial fluid lymphocytes from patients with osteoarthritis did not express either marker. The expression of these markers demonstrates the presence of potentially functional cytolytic lymphocytes, expressing proteins required to mediate killing, in the synovial fluid of patients with rheumatoid arthritis. This suggests that cytolytic lymphocytes may be involved in the pathogenesis of rheumatoid arthritis.