Summary: Natural hemolysins in human sera are more resistant to heat and age than complement. There is no relation between the complement content and natural hemolysins in human sera. Absorption of active human sera with corpuscles removes hemolysin but not complement. Washed sensitized cells have not absorbed complement and do not undergo hemolysis unless complement is furnished. Filtration may remove complement from a serum without any or but slight removal of a natural hemolysin. Heating serum sera at 56°C. for thirty minutes results in the partial destruction and inactivation (“masking”) of natural hemolysins, the different hemolysins varying in their resistance; heating at 62°C. results in the destruction of natural hemolysins.