The present study describes the respiratory immune response of mice to locally administered antigen, and the modulation of this response by systemic immunization. Intranasal immunization of mice with the A/PR/8/34 strain of influenza virus evoked local antibody response of the IgA type. The titer of the IgA antibodies declined to a nondetectable level in 40--50 days. If at that time a second intranasal dose was administered, a secondary IgA response was evoked. On the other hand, administration by the intramuscular route resulted in a mixed population of IgA and IgG antibodies. The relevance of this finding to problems of immunization against respiratory viral infections is discussed.