1. A study has been made of the effect on the isolated frog's heart of a preparation derived from the supernatant fluid of cultures of hemolytic streptococcus.
2. The streptococcal preparation was found to induce systolic contracture, the contracture usually developing only after the second of two administrations of the preparation. It was found that a single application of the streptococcal preparation sensitized the heart to a second application.
3. The properties of the cardiotoxic factor so far as they have been studied were found to be identical with those of the oxygen-labile hemolysin of streptococci. The capacity of normal and of immune sera to neutralize the cardiotoxic action paralleled the antihemolytic potency of the sera.