Influenza virus infection or vaccination evokes an immune response to viral hemagglutinin HA and neuraminidase NA surface glycoproteins, which results in neutralizing antibody against influenza A that is subtype or even strain specific, but not broadly protective. A heterosubtypic immune response that offers protection against avariety of different influenza A subtypes has been induced in experimental animal models, and there are epidemiologic data that suggest a degree of protection may occur in humans. Early studies of this broad cross-protective response indicated that cytotoxic Tlymphocytes CTL were responsible. However, more recent studies in mice demonstrate that antibodies also contribute to this immune response. We established a model of heterosubtypic immunity HSI in cotton rats Sigmodon hispidus that offer a number of advantages to study influenza pathogenesis and immunity to influenza. Cotton rats were infected with influenza APR834 H1N1 or AWuhan35995 H3N2, and then challenged with AWuhan35995H3N2 virus 4 weeks later. The results demonstrated measurable heterosubtypic immunity in cotton rats, characterized by enhanced viral clearance, protection from tachypnea a reliable measure of lower respiratory infection in this model, a vigorous early cellular recall response, and a reduction in bronchiolar epithelial cell damage. Further studies to elucidate the contribution of antibody to this response showed that cotton rats transfused with H1N1-immune serum prior to challenge with an H3N2 virus were protected from influenza-associated tachypnea, and the quality of protection correlated well with the antibody titer transferred. Immunization with an inactivated preparation of virus delivered intramuscularly also provided protection, suggesting that cell mediated andor mucosal antibody may not be required for protectionin our model.