Lactic acid concentrations in the synovial fluid of 84 patients with acute monoarticular arthritis were determined by gas liquid chromatography. Lactic acid values in 27 cases of nongonococcal septic arthritis were strikingly higher (mean 1170 mg/100ml) than in 45 cases of inflammatory or degenerative arthritis (mean 34 mg/100 ml), as well as in 12 cases of gonococcal arthritis (mean 27 mg/100 ml). With the proper equipment, determination of lactic acid can be a relatively rapid, reliable procedure. Synovial fluid lactic acid concentrations therefore can be used as a rapid, supplemental diagnostic aid in differentiating nongonococcal septic arthritis from both gonococcal and nonseptic acute arthritis.