AbstractThe spleens of rats were studied by light and electron microscopy during the course of an acute myelogenous leukemia, with special reference to infiltration of leukemic myeloblasts in the spleens and to the correlation of leukemic cell infiltration with splenic hematopoiesis and splenomegaly. Leukemic myelo‐blasts infiltrated the cordal space of the red pulp. Many of them appeared in groups. Even in spleens which were heavily infiltrated, leukemic myeloblasts did not penetrate the white pulp. Massive infiltration and proliferation of the leukemic myeloblasts in the red pulp resulted in splenomegaly. The spleen increased its he‐matopoietic activity, while the medullary hematopoiesis diminished due to the invasion of leukemic myeloblasts in the bone marrow. Compensatory splenic hemato‐poiesis occurred in most of the leukemic spleens, but diminished in spleens which were very heavily infiltrated with leukemic myeloblasts. Thus, the degree of splenomegaly and splenic hematopoiesis did not necessarily correspond to the percentage of leukemic myeloblasts in the bone marrow, but rather related to the number of leukemic myeloblasts present in the spleen. A possible role for the splenic sinus walls in promoting compensatory hematopoiesis in the spleen is discussed. A consistent association of type “C” virus particles with leukemic myeloblasts was