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.