Organ cultures of rhesus thyroid tissue supported the growth of mumps virus over a period of 12 days of incubation. Growth of virus was demonstrated by increasing titers of infectious mumps virus in cultures harvested at various times after inoculation. Virus was released into the fluid medium and this release was demonstrated to be continuous process. Mumps virus antigen was demonstrated in the cytoplasm of follicular cells and possibly in the lumen of follicles themselves by the fluorescent antibody method.