Growing Wistar rats (30 animals) were divided into 3 groups of 10 each and fed semisynthetic diets: control (C), high-fat (67%) containing soyabean oil (O) or lard (L). After 5 weeks the animals were anesthetized and blood was sampled for analysis of nonspecific immunity indicators, antioxidant enzymatic activity, and blood count. Diet L (lard) led to increased activity of stimulated neutrophils and monocytes. The oxidative burst of neutrophils, activated by E. coli, was stimulated by the consumption of an energy-dense diet (lard and oil vs control), whereas the number of phagocytosing cells with reactive oxygen species was smaller in the group of animals fed lard. Moreover, disorders of redox homeostasis were found; diminished superoxide dismutase activity was accompanied by increased GPx activity in rats receiving a high level of dietary fat.