1. A specific antibody, demonstrable by the technique of complement fixation, regularly appears, or increases in concentration, in the sera of human beings during an attack of mumps or during convalescence.
2. Specific dermal hypersensitivity, demonstrable by the injection of heat-inactivated mumps virus, has been shown to develop in 6 human beings after recovery from mumps.
3. Complement-fixing antibody and the hypersensitive state also emerge as a result of clinically inapparent infection with the virus of mumps.
4. These two phenomena are apparently unrelated in respect to immunologic mechanisms.
5. The data presented indicate that the complement fixation test should prove of value both in diagnosis and in the determination of immunity.
6. The skin test for dermal hypersensitivity, on the other hand, becomes positive after recovery and therefore would appear to be useful only as an index of resistance.