Summary: Eosinophilia was induced in the peritoneal cavity of guinea pigs by the passive transfer of antigen-antibody complexes. These results were obtained in three systems: bovine serum albumin and rabbit antibody, hemocyanin and either guinea pig or rabbit antibody. Complexes were effective whether prepared in antigen excess, antibody excess or at equivalence. Activity disappeared from the antisera when the antibody was absorbed by the specific antigen. These experiments identify the active factor in serum as antibody and provide direct evidence that eosinophilia is induced by immune complexes. The conclusion is reached that eosinophilia is one of the consequences of antigen-antibody union and that eosinophils do not mediate the production of antibody. The possibility that a common mechanism underlies the eosinophilia which is noted in diverse pathologic states is discussed.