The microrheological characteristics of reticulocytes in vivo was studied. Method of anemia in rabbits induced by ⁶⁰Co radiation was used. The measurements of percent of reticulocytes, deformation index, orientation index, etc in the processes of reticulocytes changing into erythrocytes in vivo for 72 h were performed. It was shown that there were obvious changes in the reticulocytes rheological characteristics in the courses of this changing. Therefore, the found had some basic theoretical and clinical significance.