Human sera taken at various stages of mumps have been analyzed in regard to their reactivity with two serologically distinct complement fixation antigens which were derived from the infected chick embryo. Antibodies to the soluble or S antigen appear earlier in the disease and, as a rule, reach high levels before antibodies against the virus-bound or V antigen commence to rise. In early convalescence, both antibodies reach high levels. Subsequently antibodies against the S antigen decrease usually at a faster rate than those against V, so that after a period of several years, frequently only anti-V may be left. These findings were found helpful in diagnostic procedures. The use of both the V and S antigens has permitted the early diagnosis of manifestations of mumps in the absence of parotitis, such as meningoencephalitis, since the finding of high levels of anti-S and of low titers of, or no, anti-V is considered diagnostically significant for the first few days of illness. For the determination of resistance the use of the V antigen appears more useful. Following vaccination or skin testing, antibodies against both antigens may develop; those against V increase more regularly and to higher titers than those against S.