We determined lactic acid levels by the lactic dehydrogenase method in synovial fluid of 41 patients with various rheumatic diseases, to test the concept that significantly elevated values were diagnostic of septic arthritis. Nine patients had septic arthritis, 15 rheumatoid arthritis (RA), and the remainder miscellaneous conditions. In another 9 patients with different rheumatic diseases, including 1 with septic arthritis, synovial fluid lactic acid was determined by both the lactic dehydrogenase and gas-liquid chromatography methods. There was a wide scatter of values among patients with septic and nonseptic inflammatory arthritis, and much overlap occurred. We could not differentiate septic arthritis from RA on the basis of synovial fluid lactic acid levels. Results were similar with both procedures for determining lactic acid levels.