Summary1. 180 determinations of antistreptolysin level in serum were performed in 104 patients with various diseases. This research attempted to assess the diagnostic value of the test.2. In most cases of rheumatic fever, the antistreptolysin level is increased; it is usually normal in chronic polyarthritis.3. In the other joints diseases (tuberculous, gonococcal arthritis,…), there is usually no increase in the antistreptolysin level.4. Antistreptolysin level is increased in acute nephritis following streptococcal infection ; it is normal in chronic nephritis.5. It is high after angina and erythema nodosum of streptococcal origin.6. The rise of antistreptolysin above normal may be considered as characteristic of an hemolytic streptococcal infection.