Rheumatoid arthritis is a chronic autoimmune joint disease of unknown etiology. T cells are believed to be important in the pathogenesis of rheumatoid arthritis since they infiltrate the joints and express several activation markers, such as MHC class II and IL-2R. In this study we have elucidated the effect on freshly isolated T cells of rheumatoid arthritis synovial fluid (RA-SF), which contains in vivo produced cytokines and enzymes. The mouse mixed lymphocyte culture (MLC) has been used as a model and specific cytotoxicity was evaluated against 51Cr-labelled sensitive target cells. Studies have shown that RA-SF contains a B cell differentiation activity that can cross-react between the human and murine species. Here we have shown that the addition of RA-SF strongly potentiates cytotoxic activity as well as lymphokine production by allogeneic activated effector T cells. The enhanced cytotoxicity induced by RA-SF was found to be due to a combined effect of increased cytotoxic T lymphocyte (CTL) precursor frequency, measured by limiting dilution analysis, and a more efficient killing on a per cell basis. Kinetic studies show that RA-SF must be added within 48 h after initiation of the MLC, otherwise the effect is lost. The target cell specificity of RA-SF was studied, using enriched CD4+ or CD8+ responder cells in the MLC. It was found that RA-SF could act directly on the CD8+ cells and potentiate their development to cytotoxic effector cells: this activity was not found when CD4+ responder cells were used instead. RA-SF could, on the other hand, greatly enhance IL-2 production by CD4+ responder cells. We suggest that B and T cell activity in RA-SF is important in the propagation of chronic inflammation in the joints of patients with rheumatoid arthritis.