Vaccine formulations of virulent, modified/attenuated, or inactivated mono- or multivalent types of chicken embryo-propagated avian infectious bronchitis virus (IBV) at different passage levels and administered by non-parenteral routes have been used for immunization of chickens against the disease. Two doses of vaccines are generally used: 1) high level virus of lessened virulence, antigenicity and immunogenicity as "derived" from its parental virus via serial passage in and selective adaptation to the chicken embryo, and 2) low passage level virus with the above properties more closely related to the greater potency of the "primordial" parental virus in nature. The efficacy of vaccines with contemporary or candidate virus has been interpreted from a variety of criteria among which are: 1) clinical response, residence and dissemination of virus, 2) in vivo and in vitro local tissue response of tracheal epithelium, 3) protection against virulent homologous or heterologous types of IBV, and 4) induced secretory and humoral antibody. Vaccines have been economically useful and of epizootiological value but the greatest difficulty is the uncertainty of protection because of the changing antigenic, immunogenic and other properties of existing virus and of newly emerging types. With a complex virus such as IBV, the opportunities for natural selection and evolution of new or modified antigens via association with antibody of immune or of partially immune hosts are as enormous as antigenic lability resulting from fragmentation of the primordial genome under natural and/or artificial environmental stress. The present day IBV might be progeny from a unique pool of primordial genome via mutation and host-induced variation, or even persistence of primordial virus as an infinitesimal portion of the population.