After a single 5-min exposure to an aerosol containing ovalbumin, rats produced a heat labile, skin sensitizing antibody by 14 days. By radioiodination of the protein the amount of antigen deposited in the respiratory tract during this immunization was calculated to be 12.8 µg. The amount of protein swallowed by the animals (80 µg) was incapable of elaborating an IgE response. The synthesis of IgE under these conditions was absolutely dependent on heat-killed Bordetella pertussis vaccine given i.p. before exposure. Second exposures to antigen resulted in renewed and accelerated IgE production. Repeated aerosolization with a low dose of antigen (0.1%) caused no detectable antibody synthesis.