Presence of muramic acid, a bacterial cell wall component, was analysed by gas chromatography-mass spectrometry in synovial fluid (SF) of 40 patients with acute inflammatory arthritis. SF muramic acid was observed in 4/14 patients with acute, culture negative inflammatory arthritis of unclear origin. Each of these four patients had a history of a recent bacterial disease (pansinuitis, purulent leg ulcer, erysipelas, unexplained fever with suspicion of cholecystitis and urinary tract infection). In the bacterial arthritis, SF muramic acid was detected in 6/12 patients (in 2/6 culture negative cases). In the reactive arthritis due to Salmonella or Yersinia, the rate of positivity was 2/14. Nineteen samples of traumatic SF effusion were muramic acid negative. These findings indicate that several cases of undefined acute inflammatory arthritis are of bacterial origin.