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 α (TNFα). Seven of 12 sera from RA patients contained TNFα, while only 1 of those from reactive arthritis patients was positive. Gamma-inter-feron was detected in the synovial fluids and sera of only the RA patients. Tumor necrosis factor β was not detected in any sera or synovial fluids. RA patients with detectable TNFα had higher erythrocyte sedimentation rates and synovial fluid leukocyte counts.