Infectious bronchitis virus (IBV) causes severe respiratory tract infections in young chickens and has significant negative impact on the poultry industry. Information on the functional roles of macrophages in response to IBV infection is scarce. Objectives of the work described in the thesis were to 1) establish an effective means of depleting macrophages within the respiratory tract of chickens using clodronate encapsulated-liposomes, 2) determine the effects of IBV infection on macrophage numbers in vivo, and 3) the effect of macrophage depletion on the replication of IBV and resulting pathogenesis. We found that clodronate encapsulated-liposomes were effective in significantly depleting macrophages in the respiratory tract. We also observed that an infection with the Conn strain IBV results in an increase in macrophage numbers in trachea and lung, and that clodronate encapsulated-liposome treated trachea have a higher IBV genome load at the beginning of infection when compared to untreated tissues. Our data suggests that respiratory macrophages may play an important role in limiting the replication of IBV within respiratory tissues.