Tumor necrosis factor alpha and interleukin 1 beta concentrations were measured in synovial fluid of 24 infants and children with diagnoses of suppurative arthritis (n = 16) and other kinds of arthritis (n = 8). Large concentrations of tumor necrosis factor alpha (range, 100 to 85,000 pg/mL) were found in 12 (75%) of 16 patients with bacterial infection and in none of the patients with noninfectious origins. Large concentrations of interleukin 1 beta (greater than 200 pg/mL) were found in 15 (94%) of 16 patients with bacterial infection and in none of the other patients. In the latter group, small concentrations of interleukin 1 (range, 40 to 120 pg/mL) were present in 5 (63%) of 8 patients. Serum samples obtained simultaneously were negative for both cytokines. Tumor necrosis factor alpha and interleukin 1 beta concentrations correlated significantly and with leukocyte counts in synovial fluid. We conclude that large concentrations of tumor necrosis factor alpha and interleukin 1 beta are produced locally in patients with suppurative arthritis and they may be potentially useful in differentiating this condition from other kinds of arthritis.