Chickens fed the androgen analog mibolerone during the first 7 weeks of life regress their bursa of Fabricius but can be properly immunized by vaccination against avian pathogens of major economic importance such as Newcastle disease virus, infectious laryngotracheitis virus, avian encephalomyelitis virus, infectious bronchitis virus, fowl pox virus, Marek's disease virus, and Pasteurella multocida, the pathogen causing fowl cholera. These findings on immunocompetence to infectious agents are important because we have previously shown that the administration of mibolerone prevents the development of lymphoid leukosis tumors.