Summary and Conclusions: The antihyaluronidase content of sera from 335 normal subjects and individuals with non-infectious and non-streptococcal infectious diseases has been determined by means of the mucin-clot prevention test, using hyaluronidase produced by a strain of group A, type 4 hemolytic streptococcus as a source of enzyme. The antihyaluronidase titre at birth corresponded with that of the mother. The mean titre soon began to decrease and in the age group, 6 months, reached a level too low to be measured by the M.C.P. test. In the 5 year age group the mean antihyaluronidase titre began to increase and in the 20 year age group attained a level which was maintained throughout adult life until the age 60 when a slight, yet significant, decrease in titre was noted. The mean antihyaluronidase titre did not decrease further. Comparison of the antihyaluronidase titres of sera from so-called normal individuals and subjects with non-streptococcal infectious diseases revealed similar titres at comparable ages. Addenum: Since this study was completed, Pike (26) has described a test by means of which 55% of the 110 strains of non-capsulated group A streptococci were shown to be hyaluronidase producers. The use of this test may make it possible to demonstrate hyaluronidase in strains which were formerly thought to be non-hyaluronidase producers. It may be that a rise in antihyaluronidase titer against streptococcal hyaluronidase is always preceded by an infection with a hyaluronidase-producing strain of group A hemolytic streptococcus. If this proves to be the case, it will be further evidence in favor of the specificity of streptococcal hyaluronidase.