Sera obtained from patients at the time of acute active infections were found, in every instance, to be bactericidal for hemolytic streptococci. The observations were made with sera from twenty-five patients. The group consisted of cases of pneumococcus, hemolytic streptococcus, staphylococcus, meningococcus, tubercle bacillus, and malarial infections; the etiology of the diseases in other patients was either uncertain or may have been a mixed infection.

In sera taken from the same group of patients, soon after recovery or marked improvement had taken place, the streptococcidal property was absent or greatly diminished in potency.

By comparative tests made with sera from twenty healthy adults, the streptococcidal action was not demonstrable.

Three different strains of Streptococcus hemolyticus of the beta type were employed in the experiments. Differences in susceptibility of the strains to the killing power of the serum were noted.

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