Using Rauscher leukemia virus, changes in the reticuloendothelial phagocytic activity were studied by a carbon clearance method and a heat-damaged (51)Cr-labelled red blood cell method. With the carbon clearance method, after virus infection, phagocytic activity was slightly increased in the early stages, but decreased gradually during later stages. With the (51)Cr-labelled RBC method, in the early stages, phagocytic activity of liver and spleen was slightly increased and during the later stages accelerated markedly in the liver in contrast to supressed activity in the spleen. It was suggested that an early increase of phagocytic activity observed by both methods was a response to viral infection. Significantly accelerated phagocytic activity seen in the liver during later stages with the (51)Cr method may have been related to the hemolytic anemia associated with Rauscher leukemia.