SummaryStudy of the complement-fixation antigens obtained from chick embryos infected with mumps virus revealed 2 anti-genically distinct components separable by high speed centrifugation. One antigen is closely linked with the virus and found predominantly in the allantoic and amniotic fluids; the other component is smaller and present mainly in the allantoic and amniotic membranes. The 2 antigens can be differentiated by serum absorption technic. Some human sera contain antibodies to the 2 antigens (early convalescent sera) whereas others may show antibodies only to the virus-bound antigen but not to the soluble material (long past infection).