A double tracer study on erythrokinetics was carried out with radioactive iron and chromium in 42 patients having various diseases; 10 with iron deficiency anemia, 9 with aplastic anemia, 12 with leukemias and II with liver cirrhosis acompanied by splenomegaly. The sites of the red cell production and destruction were determined with the body surface counting techniques. The true blood volume was calculated and the whole body hematocrit correctio factor was also evaluated. RESULTS : 1. In iron deficienty anemia, all of the plasma iron turnover, red cell iron utilizationm red cell iron turnover, and the red cell iron renewal rate were accelerated. 2. In aplastic anemia, the plasma iron disappearance wa prolonged while the plasma iron turnover was normal or slightly increased due probably to a relative increase of the plasma volume and high plasma iron level. 3. In leukemias, there existed no specific pattern according neither to cell types nor to the time of onset. 4. In liver cirrhosis with splenomegaly, the plasma iron turnover, red cell iron utilization, red cell iron turnover and the red cell iron renewal rate were increased. 5. The hematocrit correction factor in patients without splenomegaly was normal while it in patients with splenomegaly was increased by about 12%.