The authors investigated 55 determinations of synovial lactic acid in 7 groups of patients: septic arthritis, seropositive rheumatoid arthritis, seronegative rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitic arthritis, reactive arthritis, mechanical fluids of osteoarthritis. Statistical analysis confirmed the diagnostic contribution of this determination in septic arthritis and demonstrated a correlation between the lactic-acid and cell levels within a cell-level range of 5,000-20,000. A significant difference was detected between seronegative and seropositive forms of arthritis. The physiopathologic interpretation of the findings in cases of rheumatoid polyarthritis and in non-rheumatoid inflammatory rheumatism is discussed.