Experiments have been reported on the response of human beings to oral spraying of attenuated active mumps virus. Among 410 children judged susceptible by negative histories and serological tests, 6 developed mild signs of infection. Up to 93% of the children in the individual groups responded with antibodies as measured by complement fixation technic. The frequency of response depended on the concentration of virus used. The preparations contained originally between 107.5 to 108.9 ID50 for chick embryos. When diluted 10- or 100-fold before spraying, the incidence of response fell to about 50% and less than 10%, respectively. Antibodies to the soluble antigen, as a rule, exceeded those against the virus antigen. Hemagglutination-inhibition tests in a small number of cases revealed formation of antibodies in 60%. In most instances the serological tests were negative one year after the spraying. However, following skin testing at that time, the antibody response was greater in these children than in comparable control subjects. Experimental exposure of 4 children to mumps virus 3 months after immunization failed to elicit the disease, whereas 4 out of 6 controls developed clinical evidence of mumps. Of the larger groups treated by this procedure only those in one institution had been definitely exposed to mumps 8 months later. Eight cases occurred among the non-immunized children, mainly new admissions to the school. Although these data permit no conclusion they justify further exploration of this method of active immunization.