Infection of rats with the parasite Mesocestoides corti increased the numbers of neutrophils, eosinophils and mononuclear cells in the blood. These peaked 11 days after infection and had declined to control levels by day 24. Increased number of eosinophils and mononuclear cells were also present in the peritoneal cavities of rats 24 days after infection. These gradually declined to reach control values by day 81. Intraperitoneal administration of glycogen to uninfected rats and to rats that had been infected for 24 and 81 days caused a transient increase in blood neutrophil numbers, maximal at 4 h. Although glycogen increased the numbers of neutrophils in the peritoneal cavities of uninfected animals and animals infected for 81 days, it did not increase the number of peritoneal neutrophils in rats that had been infected 24 days earlier. These results suggest that neutrophil infiltration can be impaired in animals undergoing an inflammatory response characterized by increased numbers of eosinophils.