The antibody response to Newcastle disease virus was monitored in the sera and salivas of adult chickens immunized by two methods: (i) combined intratracheal-intranasal vaccination followed by intratracheal revaccination or (ii) intramuscular vaccination followed by intratracheal revaccination. By solid-phase radioimmunoassay, only immunoglobulin G (IgG) and IgA antibodies to Newcastle disease virus were detected in the salivas, whereas IgA and IgM antibodies were present in egg whites. The first method produced the highest antibody levels in both serum and saliva and, in addition, prevented detectable virus multiplication in the respiratory tracts upon revaccination 4 weeks later. Plasma cells of all three classes were distributed throughout the tissues lining the oral cavities. The highest densities of plasma cells were in the Harderian glands; IgG was the predominant class, whereas IgA and IgM plasma cells were present in almost equal but lower numbers. The Harderian plasma cells were the most likely source of the antibody found in saliva.