Changes in the absolute number of splenic cells of mice infected with Rauscher leukemia virus (RLV) and a day later--with living Brucella (BA) were studied. The increase in the absolute number of splenic cells of mice infected with RLV + BA was initially caused by the lymphoid tissue reaction to Brycella and then by the development of leukemia. The number of reticular cells in the spleen appeared to be larger than the control (RLV) within all observation periods; the number of basophilic normocytes amounted to the control, but an increase in the erythroblasts number was noted a week later and reached the control level by the 30th day, practically being unchanged subsequently. In mice receiving RLV + BA there was the increased monocyte, plasmatic cell, megakaryocyte, myeloid series cell count. The shift of splenic granulocytes to the "left", the development of leukocytosis, blood neutrophilia, in particular, lagged 1--2 weeks behind in such mice. The number of normocytes getting into the blood was decreased. All this would result in a significant extension of the lifespan in mice RLV + BA.