To understand further the hematopoietic dyscrasias induced by a variant (a) of Rauscher leukemia virus (RLV), we used Escherichia coli endotoxin to stress the hematopoietic system of control and RLV/a-infected BALB/c mice. During the preleukemic stages of virus infection, there was slight splenomegaly without peripheral blood erythroblastosis. Granulocyte release and tissue mobilization mechanisms appeared unaffected by the RLV/a infection. Both RLV/a-infected and control mice reacted to endotoxin with peripheral granulocytosis and peritoneal granulocyte mobilization, though the circulating granulocyte levels in RLV/a-treated mice initially were lower than those in controls. Spleen of RLV/a-infected animals were larger than those of controls, but both responded to endotoxin with elevated numbers of granulocytes and erythroblasts. Since numbers of bone marrow erythroblasts in both groups of mice were decreased after endotoxin, stem cell competition and/or shunting of stem cells from marrow to spleen may have been involved. Endotoxin also induced rapid falls in hematocrit levels in both groups. These studies suggested that RLV/a-infected mice can be a model to study 1) erythropoietic dysfunction uncomplicated by defective granulopoietic release and tissue mobilization control mechanisms, and 2) progression of evolving granulocytic leukemia.