Eight groups of chicken immune sera and one group of turkey immune sera were used to identify unique in vivo-expressed antigens of Pasteurella multocida by Western immunoblotting. When antisera were prepared against the detergent-insoluble fraction of in vivo-grown P-1059, live X-73, live P-1059, live 86-1913, live P-1662, and live P-1702, they recognized four major antigens on immunoblots of the detergent-insoluble fraction of in vivo-grown serotype 1 and in vivo-grown serotype 3. These four antigens, which have approximate molecular weights of 204,000, 192,000, 179,000, and 153,000, were not recognized by antisera made against the detergent-soluble fraction of in vivo-grown P-1059, whole in vitro-grown inactivated X-73, or whole in vitro-grown inactivated P-1059. The results provide evidence for common antigens expressed by in vivo-grown serotype 1 and in vivo-grown serotype 3. These antigens are recognized by sera from chickens that have survived infection by various serotypes. The antigens are concentrated in the detergent-insoluble fraction of in vivo-grown bacteria and likely represent unique in vivo-expressed outer-membrane proteins.