AbstractWe have previously shown that increased resistance toSalmonella enteritidisorgan infectivity in day‐old chicks was conferred by the immunoprophylactic administration ofS. enteritidis‐immunc lymphokines (ILK). This resistance was associated with a significant increase in the number of circulating heterophils 4 h after ILK injection. The objective of the present study was to evaluate heterophil function following the administration of ILK in day‐old chicks. Significant increases (P<0.001) in adherence, chemotaxis, and phagocytosis ofS. enteritidiswere found with heterophils isolated from ILK‐injected chickens compared to the heterophils isolated from birds injected with either pyrogen‐free saline or lymphokines from non‐immune T cells. After phagocytosis, the heterophils from the ILK‐injected chickens were also able to kill significantly greater numbers ofS. enteritidismore rapidly than did the heterophils from the saline‐injected control birds (within 30 min, control cells killed 21.89% of the bacteria whereas ILK‐treated cells killed 88.22%). We also found that the heterophils from the ILK‐injected birds were more efficient killers ofS. typhimurium, S. gallinarum,andE. coliThese results strongly suggest that the protection againstS. enteritidisorgan invasion induced by the prophylactic treatment of day‐old chicks with ILK involves activated heterophils which migrate rapidly to the inflammatory stimulus where they phagocytize and kill the bacteria.J. Leukoc. Biol.57: 56–62; 1995.