1. A chemically pure hemolysin-leucocidin has been isolated from ß hemolytic streptococci, but not from other species of bacteria studied.

2. It does not give rise to antibodies, but precipitates immune sera against hemolytic streptococci, and is therefore a hapten.

3. A highly purified sample of S. H. up to a dilution of 1:128,000 hemolyzes red blood cells. Its hemolytic activity is not specifically neutralized by antiserum versus ß hemolytic streptococci. It is leucocidic in that it inhibits the reduction of methylene blue by leucocytes.

4. The hemolysin-leucocidin is stable to oxygen, to heat and to moderate changes in hydrogen ion concentration. Its chemical structure has been determined in part. Its molecular weight is 2260.

5. A crystalline derivative has been isolated as the sodium salt from the hemolysin-leucocidin. As the free acid it has a molecular weight of 720. Its hemolytic and leucocidic activity parallels that of S. H., although it is not serologically active. It possesses a high degree of toxicity for mice and rabbits.

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