Various strains of infectious bronchitis virus (IBV) cause respiratory, kidney, enteric and reproductive illnesses in chickens, especially in newly hatched chicks. Assays have been developed to identify Gray strain IBV-specific cytotoxic T lymphocyte (CTL) responses using viral infected antigen presenting cells (APC) and using the Semliki Forest virus vector infected APC expressing individual viral polypeptides. It was shown that major histocompatibility complex restricted CTL are responsible for early control of IBV infection. The kinetics of viral load observed in the lungs and kidneys correlated with the level of IBV-specific CTL activity of effector cells prepared from spleens of infected chicks. Adoptive transfer of immune T cells to chicks prior to infection demonstrated that IBV primed CD8(+), alphabeta T lymphocytes could protect chicks from acute infection. CTL determinants in the viral particle can be mapped to the spike and nucleocapsid proteins but not to the membrane protein. The carboxyl terminus of the nucleocapsid protein houses an epitope(s) responsible for induction of CTL responses to IBV N protein. Inoculation of DNA plasmid expressing the carboxyl terminus of Gray strain N resulted in induction of CTL that cross-react with two distinct IBV strains. In addition, this potential DNA vaccine resulted in protection of chicks against acute infection.