Heterophils, the functional equivalent to the mammalian neutrophil, are important mediators of natural resistance against invasive pathogens in poultry. Young poultry are susceptible to pathogens, such as Salmonella enteritidis, during the first week post-hatch. No studies have evaluated the ontogeny of heterophil function in turkeys during the first few weeks post-hatch. Previous studies from our laboratory have shown day-old poults were protected against S. enteritidis organ invasion following immunoprophylactic administration of chicken S. enteritidis immune lymphokines. Therefore, the objective in the present study was to characterize the development of phagocytosis and bacterial killing by turkey heterophils during the first 3 weeks of life and to compare the effect of immune lymphokines on the development of heterophil phagocytosis and killing during the first 3 weeks post-hatch. Both functional phagocytosis and killing activities were age-dependent events. During the first 1-7 days post-hatch, little functional activity was demonstrated which apparently is associated with susceptibility. Optimal heterophil phagocytosis and killing activities were reached 14-21 days post-hatch. Administration immune lymphokines significantly potentiated phagocytosis (P < 0.01) and killing (P < 0.001) by turkey heterophils. In fact, immune lymphokine administration to 1-7-day-old poults augmented phagocytosis and killing activities of heterophils equivalent to levels found in functionally mature 14-21-day-old poults. These results demonstrate the ontogeny of the functional activity of the turkey heterophil is an age-related phenomenon, with inefficient phagocytosis and killing during the first week post-hatch. Prophylactic administration of immune lymphokines significantly potentiated the functional activity of the heterophil poults during the first 3 weeks of life. Most importantly the administration of immune lymphokines enhanced the functional activity of heterophils from 1-7-day-old poults to levels comparable to that of an immunologically mature bird.