In experiments analyzing dose-response, intratracheal inoculation of chickens with 10(8) and 10(9) avirulent Pasteurella multocida organisms induced the migration within 24 hr of large numbers of respiratory lavage cells (RLC) with increased phagocytic proportions and phagocytic capacity. Doses from 10(4) to 10(7) organisms per bird resulted in elevated numbers of RLCs that were not significantly higher (P > 0.05) than values of uninoculated or mock-inoculated control chickens. When analyzing the duration of response, we found that inoculation with 10(9) organisms resulted in significantly higher (P < 0.05) numbers of RLCs for 63 to 89 hr without significant elevation in phagocytic proportion and capacity. The numbers of RLCs were elevated, although not significantly (P > 0.05), up to 11 days after inoculation. These results indicate that RLCs migrate to the respiratory tract only in response to a relatively high number of stimulating bacterial organisms and that the duration of response is relatively short. Although there were elevated numbers of RLCs beyond 89 hours after stimulation, the question remains as to whether these cells would enhance nonspecific defense of the respiratory system of chickens.