Pyrogenes Salinem antigen, prepared at different temperatures, is used to adsorb heterologous anti-Chiffon and anti-Pomona sera, and the lability point of the antigen is determined by the macroagglutination test. Results show that the effect of this thermolabile antigen starts to be destroyed after 10 min at 60 degrees C with anti-Pomona serum and at 65 degrees C with anti-Chiffon serum. The method of measurement is not sensitive enough and the lability point would probably be lower than that obtained in these tests. The authors consider a more accurate method to be the combined use of the adsorption and microagglutination tests, with comparative titration of an heterologous antiserum.