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Effects of iron dextran injections on anemia developed in theileriosis were evaluated in calves experimentally infected with Theileria sergenti . Splenectomized animals were inoculated with parasitized erythrocytes from donor animals, and the course of anemia, methemoglobin concentrations of erythrocytes (MetHb, an indicator for blood oxidation), serum iron and erythropoietin concentrations were determined. At the time that packed cell volume (PCV) became 20-25% , 23-41 days after infection, iron dextran (1g of iron per day) was intramuscularly injected into animals for 3 days and 10 days in the separated trials. The course of anemia in a treated group was compared with that in each control group with no treatment. In the 3-day treatment trial, no statistical differences were detected in the state of anemia between the treated and control groups at any sampling time points. However, in the 10-day treatment trial, the treated group showed higher erythrocyte counts in the peak stage of anemia 2-3 weeks after initiation of treatment, and higher PCV and hemoglobin levels in the convalescent stage of anemia 6-8 weeks after initiation of treatment, than the control group. MetHb and serum erythropoietin levels increased transiently and similarly in the two groups at the time of developing anemia. The treated group showed higher serum iron levels 1 and 7-10 weeks after initiation of treatment than the control group. The present study indicated that the 10-day treatment with iron dextran in the developing stage of anemia was effective to suppress the reduction of erythrocytes in the peak stage of anemia, and to improve the recovery of PCV and hemoglobin levels in the convalescent stage of anemia caused by T. sergenti infection., 小型ピロプラズマ病貧血に対するデキストラン鉄製剤（鉄剤）の効果を解析した. 摘脾牛に原虫寄生赤血球を接種して感染させ, 貧血経過, 赤血球メトヘモグロビン濃度（MetHb, 血液酸化指標）, 血清鉄, エリスロポエチン濃度を測定した. 原虫感染後23～ 41 日に血球容積（PCV）が20 ～ 25％となった時点で, 投与群に鉄剤1g を3 日間, あるいは 10 日間筋肉内に投与する試験を個別に実施し, それぞれの非投与の対照群との比較を行った. その結果, 3 日間投与試験では貧血経過に群間の差はみられなかった. しかしながら10 日間投与試験では, 貧血極期となった投与開始後2 ～ 3 週の赤血球数, および貧血回復期である投与開始後6 ～ 8 週のPCV およびヘモグロビン濃度が投与群で大きかった. MetHbは両群同様の一過性増加を示し, 投与群特有の増加はみられなかった. 血清鉄濃度は投与開始後1 週と7 ～ 10 週に投与群で大きかった. 血清エリスロポエチン濃度は両群で貧血進行期に同様に増加した. これらの結果から, 原虫実験感染牛の貧血進行期に鉄剤を10 日間投与することにより, 貧血極期の赤血球数減少の緩和と貧血回復期のPCV およびヘモグロビン濃度の改善に効果があることが検証された.