Synovial fluid lactic acid estimations were carried out on 50 samples by gas liquid chromatography. Specimens from 4 patients with bacteria arthritis, other than gonococcal, had a mean lactic acid concentration of 215 mg/dl. One patient with gonococcal arthritis had a synovial fluid lactic acid of 30 mg/dl. Forty-one patients with inflammatory arthritis and 4 patients with degenerative arthritis had mean synovial fluid lactic acid levels of 27 and 23 mg/dl respectively. The estimation of synovial fluid lactic acid is reliable in differentiating septic arthritis from inflammatory and degenerative arthritis except when the infecting organism is NEisseria gonorrhoeae.