A significantly increased spontaneous cell-mediated cytotoxicity (SCMC) has been reported in synovial fluid lymphocytes (SFL) as compared to peripheral blood lymphocytes (PBL) of patients with rheumatoid arthritis (RA) and that of normal controls [1-3]. To determine whether this increased SCMC activity is due to the production of a lymphokine and related to the production of a lymphotoxin(LT)-like mediator, PBL from normal controls and PBL and SFL from RA patients were incubated either with a human melanoma cell line (IGR 3) or with cell-free synovial fluid (SF) from RA patients. The SF and the cell-free supernatants of the different cultures were tested for LT activity by estimation of inhibition of DNA synthesis of HeLa cell monolayers and they were added to a SCMC assay system using normal PBL and IGR 3 as target. In the supernatants from cocultures of either PBL from controls or PBL and SFL from RA patients with IGR 3 cells, there was no significant difference in LT activity. An LT-like mediator was observed in the supernatants of all lymphocytes cocultured with SF, whereas SF alone and supernatants of lymphocytes alone exhibited little or no LT activity. In a control experiment, LT induction was not observed when normal lymphocytes were cultured with the serum of RA patients. Absorption of the culture supernatants with an insolubilised goat anti-human Ig did not remove LT activity. The demonstrated release of an LT-like mediator from lymphocytes incubated with SF might be one contributing mechanism to the inflammatory joint reaction in RA patients.