Lactic acid concentrations in the synovial fluid of 71 patients with inflammatory arthritis were determined by an enzyme method. In 63 samples from 54 patients with a variety of non-septic arthritides, including rheumatoid arthritis, reactive arthritis and gout, the concentration of lactic acid was never greater than 10.2 mmol/l, whereas all twelve patients with septic arthritis had concentrations of 11 mmol/l or greater. Two patients with gonococcal arthritis did not have raised lactic acid concentrations. The enzyme method of lactic acid estimation is an accurate reproducible means of differentiating septic from nonseptic arthritis prior to the isolation of the infecting organism. However, caution is necessary when interpreting the results in those patients who have recently received antibiotic therapy, or in whom gonococcal arthritis is suspected.