The development of mumps saliva antibody and its relationship to oral virus excretionwere studied in the cases of natural infection. In most cases, saliva IgA antibody began to be detected from the 4 th day after the onset of disease, and its titer reached 1:8 or below by indi-rect immunofluorescent technique. IgM antibody also appeared almost at the same time to reach a titer of 1:4 or below, but its duration was shorter than that of IgA antibody. Mumps virus was isolated from saliva until the 5 th day after the onset of disease. In many cases the appearance of IgA fluorescent antibody in saliva coincided with the termination of virus excretion. The antibody produc-ed in serum was also measured in comparison with that in saliva in four cases. The highest serum IgG antibody titers during the course ranged from 1:128 to 1:512. In these cases, however, saliva IgG antibody was not detected at a 1:2 dilution. IgA antibody reached the titers ranging from 1:8 to 1:16 in serum and the titers ranging from 1:2 to 1:8 in saliva of all these cases. Antibody activity was detected in parotid fluid specimens which had been collected directly from Stensen's ducts. From these data, it was presumed that the specific antibody was produced in salivary glands by virus proliferation, and that the antibody response of salivary glands might play an important role in the inhibition of growth of mumps virus and its excretion into saliva.