Adult hamsters inoculated intranasally with the egg-adapted Enders strain of mumps virus produced complement-fixing antibodies against both the soluble and viral antigens. The antibodies were present in measurable amts. from the 7th day after inoculation until the 38th day when the expt. was terminated. The production of antibodies to the soluble antigen was considered to be indicative of virus multiplication in the tissues of the animals. Multiplication was then demonstrated by recovery of the virus after serial passage through hamster lungs. Mumps virus has been isolated from hamster lungs following intranasal inoculation of throat washings from 4 patients with epidemic parotitis. Histopathological examination of lungs of hamsters inoculated either with the egg-adapted Enders strain or with a lung passage of a freshly isolated strain of mumps revealed the presence of bronchitis and interstitial pneumonitis. Specific antiserum when mixed with the standard virus suspension and similarly inoculated prevented such pathological changes, while nonspecific serum had no such effect.