1. The hook worm of man—both the Old and New World types—contains a weak hemolytic agent active in vitro. It is soluble in salt solution, is easily destroyed by heat and acts slowly.
2. The hemolysin is present in all parts of the worm and probably is associated with the intestinal tract.
3. The hook worm of the dog contains a similar hemolysin.
4. These hemolysins are not specific, but will act on human blood as well as on that of the dog and rat. They are only demonstrable in concentrated extracts.
5. Concentrated extracts of the human whip worm may be hemolytic to some bloods.
6. The round worm of man contains no hemolytic principle in any part of its body.
7. It seems very unlikely that this weak hemolysin found in the hook worm can have any relation to the anemia of uncinariasis.
8. Study of the "blood cysts" in the human intestine shows that the hook worm may live in a small amount of the host's blood for days without causing any marked hemolysis.