1. A factor capable of causing the hemolysis of the erythrocytes of man, chicken, and sheep occurs in the amniotic and allantoic fluids of chick embryos infected with the virus of mumps. 2. The hemolysin has not been found in normal fluids or in those infected with PR8 or Lee B strains of influenza virus. 3. The hemolysin is definitely inhibited by the serum of man and monkey convalescent from mumps, but only slightly by the serum of the acute phase. 4. The hemolysin is almost completely inactivated at 50°C. after 10 minutes. It exhibits maximal activity at 37°C. and is completely inactive at 4°C. A pH range from about 7.0 to 8.0 allows for maximum activity. 5. Adsorption and elution of the hemolysin with red blood cells has been demonstrated. After elution of the hemolysin, the red blood cells exhibit an increased osmotic fragility. Similar treatment of red cells with influenza virus did not alter this property. 6. The relationship of the hemolysin to the hemagglutinin and the enzyme-like behavior of the former have been discussed.