Two-week-old Leghorn chicks injected intravenously with 5-fluorouracil (5-FU) were found to have a three- to fivefold reduction (P < .05) in circulating polymorphonuclear leukocytes (PMN) 5 to 10 days later. The reduction in PMN was dose-dependent with a dosage of 200 mg/kg body weight of 5-FU, causing suppression of PMN to almost undetectable levels. Increasing the dosage of 5-FU to 400 mg/kg had no further effect on reducing the number of PMN in the circulation nor were overt clinical signs of toxicity observed. Single per os administration of 10(6) viable Salmonella enteritidis (SE) to individual groups of chicks treated or not treated with 5-FU at 2, 4, 6, 8, 10, or 12 days postinjection resulted in a two- to sixfold increase in the incidence of SE organ invasion (liver and spleen) beginning on Day 4 postinjection and continuing through Day 12 postinjection. The greatest increase in organ invasion occurred on Days 8 through 10 postinjection and was inversely proportional to the greatest reduction in the number of circulating PMN. Using 5-FU, a granulocytopenic chicken model was developed that can be used to study the defensive role of avian heterophils against infectious agents. The results from these preliminary experiments indicate that PMN play a significant role in reducing organ invasion by SE in Leghorn chicks.