Malignant lymphoma cells of different types were injected intravenously into intact, isologous mice. Cells designated as lymphosarcoma cells were trapped initially in the lungs or the liver after injection into a tail vein and a mesenteric vein, respectively. A few days later, however, they were distributed evenly over both the lungs and the liver, regardless of the site of injection. Within 8 days, all animals had died with greatly enlarged livers and spleens and massive numbers of tumor cells in the lungs. Cells designated as lymphocytic leukemia cells formed sharply demarcated, spherical nodules in virtually every organ investigated, regardless of the site of injection. Only in the lungs were they confined to the capillaries. Despite these differences, the interaction of the cells with the surrounding lung or liver tissue was identical for both types of tumors. In the lung capillaries, the endothelium was never injured; but, in the liver, the tumor cells quickly penetrated the sinusoidal wall by fragmentation of the endothelium over large areas, withoutthe formation of pseudopods. However, after the extravasation, pseudopods were extended into adjacent hepatocytes. The hepatocytes were often greatly distorted by the tumor cells, but they never seemed to be obliterated; the bile canaliculi always remained intact. Although the Kupffer cells were initially in close contact with the tumor cells, no phagocytosis or any other signs of activation of the Kupffer cells developed.