SummaryIt is impossible to clearly define the normal antistreptolysin O titer; in addition the great deal of non specific antistreptolysin-like activities shown by the beta-lipoprotein justifies their elimination, allowing the titration of the specific antistreptolysin. The presence of pathologic non-specific antistreptolytic titers will guide the investigator towards hepatic or renal diseases, metabolic disturbance of the lipidic fraction or tuberculosis. In doubtful cases, the reality of immunologic response toStreptococcusagression will be demonstrated by the titration of the antistrcptoprotcin-M which is quite specific and shows a better persistance than the antistreptolysin-O. This titration can demonstrate an eventual potential renal toxicity of the infectingStreptococcuswhich is coarsely typed.