The present study was aimed at elucidating the role of heterophil granulocytes during the initial infection with Pasteurella multocida subsp. multocida in chickens. Chickens (17 and 19 wk old) were depleted of their heterophil granulocytes by 5-fluorouracil treatment. When the heterophil blood counts were significantly reduced, the birds were inoculated intratracheally with 1.8–4.3 × 10⁴ colony-forming units of P. multocida. Twelve, 24, or 48 hr postinoculation, the birds were euthanatized and examined for macroscopic and histologic lesions in the lungs. Bacterial invasion was determined by culture of P. multocida from the spleen. Recruitment of heterophils into the respiratory tract during infection was found to contribute considerably to the lung lesions in chickens and was found to mediate tissue damage, possibly allowing a more rapid systemic spread of P. multocida. However, during progression of the infection, the heterophil-mediated necrosis in chickens seemed to stimulate giant cell demarcation of infected lung tissue, which coincided with the clearance of P. multocida from the spleen, thus hampering further invasion. Consequently, heterophil activation plays a dual role for the outcome of a P. multocida infection in chickens, where it initially seems to promote invasion and systemic spread but subsequently helps limit the infection by giant cell formation and bacterial clearance.