Serum and nasal wash specimens from 13 human volunteers undergoing experimental secondary infection with influenza A/Peking/2/79 (H3N2) wild-type virus were examined for the molecular form and subclass distribution of immunoglobulin A (IgA) antibodies to the viral hemagglutinin (HA). Nasal IgA antibodies were polymeric and did not bind radiolabeled secretory component, indicating that they were secretory IgA antibodies. Both IgA1 and IgA2 antibodies were detected; however, IgA1 accounted for most of the rise in IgA anti-HA levels seen after infection. In serum virtually all of the IgA HA antibodies were of the IgA1 subclass. Furthermore, the serum antibodies were predominantly polymeric and were capable of binding radiolabeled secretory component. These results suggested that the serum IgA antibodies to HA were of mucosal origin and that influenza A virus HA preferentially stimulates an IgA1 response.