A highly sensitive solid-phase radioimmunoassay has been developed for the detection of virus-specific IgA antibodies in saliva obtained from infants and children. Serial saliva samples from infants and children under 3 years of age with acute measles or mumps infection, or after measles immunization, were examined for their content of virus-specific IgA antibodies. Virus-specific antibodies appeared transiently in the saliva in both illnesses and after immunization with live attenuated measles vaccine. The significance of this finding for rapid diagnosis of these infections without the necessity of drawing a blood sample is discussed.