The pattern of colonization of an acute leukemia in rats, transferred by intravenous injection of leukemia cells, was investigated. The localization of leukemia cells and study of their proliferation in the early posttransfer stages were facilitated by labeling them in vitro with 3H-cytidine before transfer and subsequent autoradiography of sections. Immediately after injection, labeled cells were observed in the blood vessels of all organs studied. The earliest evidence of colonization was in the bone marrow and spleen. Proliferation of leukemia cells in these organscommenced within a few hours after injection. On the 3d day,deterioriation in the vascular structure of bone marrow and spleen occurred. Leukemia cells appeared in increasing numbers in the peripheral blood, and infiltration of liver, lung, lymph nodes, and thymus—only slight up to this time—became progressively more extensive. In the final stages, all organs were involved, with almost complete loss of normal architecture in bone marrow, spleen, thymus, and lymph nodes, whereas in the liver the normal architecture was maintained in large areas and the kidney became infiltrated only minimally. Although the cells transferred had already undergone neoplastic transformation, the observed growth pattern could reFlect some general aspects of the process of leukemic dissemination.