Summary Biologically, homotypic and heterotypic antisera neutralized avian infectious bronchitis virus significantly more when unheated. Morphologically, using the electron microscope technique of negative staining, there was a clear distinction between the effects of homotypic and heterotypic antisera. Heated homotypic antiserum revealed antibody attached only to the projections of the virus, while with unheated homotypic serum heat labile components could be visualized but no basic change could be seen in particle morphology. Heterotypic serum contained antibodies directed both against the projections and the envelope of the virus. In addition, unheated heterotypic antiserum produced holes approximately 100 Å in diameter in the virus membrane, suggesting that a form of virus lysis takes place. Rabbit antiserum prepared against uninfected chick-embryo fibroblasts was able to produce similar holes in the virus envelope and this led us to postulate that the envelope component of avian infectious bronchitis virus is closely related to normal chick host material.