A double tracer study on erythrokinetics was carried out with radioactive iron(Fe59) and chromium (Cr51) in 42patients having various diseases; 10 with iron deficiency anemia, 9 with aplastic aneia, 12 with leukemias and 11with liver cirrhosis acompanied by splenomegaly. The sites of the red cell production and destruction weredetermined with the body surface counting techniques. The true blood volume was calculated and the whole bodyhematocrit correction factor was also evaluated. Following were the results; 1. In iron deficiency anemia, all ofthe plasma iron disappearance, plasma iron turnover, red cell iron utilization, red cell iron turnover, and thered cell iron renewal rate were accelated. A markedly increased bone marrow erythropoiesis was observed and thered cell life span was in normal limits. 2. In aplastic anemia, the plasma iron disappearance as prolonged whilethe plasma iron turnover was normal or slightly increased due probably to a relative increase of the plasma volumeand high plasma iron level. The red cell iron utilization, red cell iron turnover and the red cell renewal ratewere decreased. A markedly decreased erythropoiesiss in the marrow with an increased extramedullary erythropoiesisin the liver was observed. The red cell life span was shortened than normal, especially in patients with bleedingtendency. 3. In leukemia, there existed no specific pattern according neither to cell types nor to the time of onset. The case of anemia in this disease seems to be a decrease in the marrow erythropoiesis or an increase of thered cell destruction with the shortened red cell life span or the combination of the both. 4. In liver cirrhosiswith splenomegaly, the plasma iron turnover, red cell iron utilization, red cell iron turnover and the red celliron renewal rate were increased. The marrow erythropoiesis was increased than normal, but a markedly increasedred cell destruction in the enlarged spleen was observed. The red cell life span was shortened than normal,especially in patients with bleeding tendency. 5. The hematocrit correction factor in patients withoutsplenomegaly was normal while it in patients with splenomegaly was increased by about 12%. The hematocritcorrection factor does not appear to be correlated with the affected disease but with degree of splenomegaly.