The immune response of mice to sublethal infection with influenza A virus was investigated by enzyme-linked immunosorbent assay, haemagglutination-inhibition and neutralization tests. The specificity and immunoglobulin class of antibodies were determined by competition enzyme-linked immunosorbent assay. A rise in geometric mean antibody levels was observed between 4 and 16 days by haemagglutination-inhibition, and between 8 and 32 days by neutralization. A positive correlation was found between the geometric mean antibody responses measured by the three assay systems, but there was no consistent relationship between the titres of individual sera by each assay method. More antibodies to the strain-specific determinants of haemagglutinin were observed 8 days after infection than antibodies to the common determinants, but by day 16 there were equal amounts of antibody to strain-specific and common determinants of haemagglutinin and to neuraminidase. Antibodies to matrix and nucleoprotein antigens were not detected until 64 days after infection. The antibody response 8 days after infection was composed of IgM and IgA immunoglobulin classes, but IgG predominated from 16 days after infection.