Rats were dosed with eggs of Taenia taeniaeformis and hematologic parameters were measured throughout the course of primary infection. There was no evidence of anemia but differential leukocyte counts revealed distinct and reproducible patterns of white blood cell changes. A lymphocytosis developed at the end of the 1st and 5th weeks postinfection (p.i.). Neutrophil counts peaked 8 days p.i., although at that time there was no marked neutrophilic infiltration of the tissues. Eosinophil counts began to rise during the 2nd week p.i., and reached a peak during the 3rd week, followed by a decline and then another peak during the 5th week p.i. Eosinophilic infiltration of the tissues was remarkable during the period of peripheral eosinophilia. A wide zone of eosinophils surrounded the developing larvae at 22 days p.i. and persisted in some cases for a further 2 weeks. Eosinophils remained in lesser numbers in the connective tissue capsule throughout the infection, often in association with plasma cells. After oral challenge with 1,000 eggs infected rats showed brisk secondary eosinophilic responses 3 to 7 days later but other hematologic parameters were unaffected. Average peripheral eosinophil counts at 3 and 4 days post-challenge were significantly higher than those in unchallenged controls (P less than 0.05 and P less than 0.01, respectively). There was no detectable increase in eosinophilic infiltration of small intestinal tissues in challenged rats. These results are discussed in relation to current understanding of the mechanisms of eosinophil chemotaxis in vitro and the possible causes of local eosinophil accumulation in parasitic infections in vivo.