Cytokines are important protein mediators in inflammatory joint diseases. The synovial fluid and plasma concentrations of interleukin-1 alpha (IL-1 alpha), interleukin-2 (IL-2), tumour necrosis factor-alpha (TNF-alpha), interferon-alpha (IF-alpha) and interferon-gamma (IF-gamma) were measured by RIA and ELISA in 28 rheumatoid arthritis (RA) patients (5 males and 23 females). Ten patients with knee effusions due to other causes (osteoarthritis, psoriasis, gout, rheumatic fever, systemic lupus erythematosus) were also studied. Eight of the RA patients had erosive disease. The synovial fluid IL-1 alpha and IL-2 concentrations were higher in Group 1 (erosive) [IL-1 alpha: 524 pg/ml (SEM: 127), IL-2: 3.28 ng/ml (SEM: 1.0)] than in either Group 2 (non-erosive) [IL-1 alpha: 241 pg/ml (SEM: 24), IL-2: 1.93 ng/ml (SEM: 0.6)] or Group 3 (non-RA) [IL-1 alpha: 267 pg/ml (SEM: 58), IL-2: 0.35 ng/ml (SEM: 0.6)] (p < 0.003 and p < 0.06 respectively). Plasma IL-1 and IL-2 levels were higher in Group 1 [IL-1 alpha: 408 pg/ml (SEM: 107), IL-2: 4.20 ng/ml (SEM: 1.5)] than in Group 2 [IL-1 alpha 150 pg/ml (SEM: 15), IL-2: 2.58 ng/ml (SEM: 0.7)] or Group 3 [IL-1 alpha: 140 pg/ml (SEM: 11), IL-2: 1.93 ng/ml (SEM: 0.3)] (p < 0.01, p < 0.009 respectively). There were no differences in the IFN-alpha, IFN-gamma or TNF-alpha levels between groups. These findings suggest that plasma cytokines levels may reflect synovial levels and that IL-1 alpha may play a significant role in erosive joint disease.