ABSTRACT. Changes in iron and ferritin in calves infected with Theileria sergenti were investigated to elucidate iron metabolism in animals with extravascular hemolytic anemia. During severe anemia, serum iron was remarkably elevated while the total iron-binding capacity remained relatively unchanged or decreased slightly in the infected calves, resulting in elevated transferrin saturation. The serum ferritin concentration gradually increased with the progress of anemia. The erythrocyte ferritin content drastically increased when mean corpuscular volume was elevated. The concentration of non-heme iron and ferritin in the liver, spleen, and bone marrow of the infected calves was markedly higher than that in the respective tissues of the control animals. In particular, the liver of the anemic calves was found to contain 23 and 35 times as much non-heme iron and ferritin, respectively, as that of the non-anemic healthy cattle. The liver type (L) to heart type (H) subunit ratio of liver ferritin was significantly higher in the protozoa-infected than in the non-infected cattle. On the other hand, the L/H ratio of marrow ferritin was significantly reduced by the anemia. These results indicate that the anemic calves infected with T. sergenti apparently present symptoms of iron overload. -KEY WORDS: anemia, calf, ferritin, iron, Theileria sergenti.