Synovial fluids from 6 of 12 patients with rheumatoid arthritis (RA) and from 3 of 11 patients with reactive arthritis contained measurable levels of tumor necrosis factor alpha (TNF alpha). Seven of 12 sera from RA patients contained TNF alpha, while only 1 of those from reactive arthritis patients was positive. Gamma-interferon was detected in the synovial fluids and sera of only the RA patients. Tumor necrosis factor beta was not detected in any sera or synovial fluids. RA patients with detectable TNF alpha had higher erythrocyte sedimentation rates and synovial fluid leukocyte counts.