Hemolytic anemia was associated with Rauscher leukemia. Anemia was observed slightly in the early stages of leukemia and developed progressively during the later stages. The mechanism of hemolytic anemia can be explained as being due to both intracorpuscular defects and extracorpuscular factors. The former, i.e., intracorpuscular defect, consists of intracorpuscular defects caused by direct effects of Rauscher leukemia virus on red blood cell and also "abnormal" erythrocytes which migrate into peripheral blood from the spleen. These "abnormal" erythrocytes were produced by the maturation of malignant erythoid cells induced by Rauscher leukemia virus. The latter, i.e., extracorpuscular factor, will be described part two of this article.