Small liver and lung metastases of the murine B16 melanoma, the tissue distribution of which was reported in a previous paper, were studied morphologically. The overall shapes of the metastases in the two organs were strikingly different: whereas liver metastases constituted sharply demarcated spheres, most lung metastases were exceedingly flat and ill defined, and covered considerable proportions of the lung surface. When studied in detail, a greater similarity emerged than was expected. Both liver and lung metastases appeared to contain small, dense tumor cell nodules, indicating a growth pattern of focal proliferation alternating with active movement. The main differences between the two localizations could be explained on the basis of preferential adhesion of the tumor cells to the pleural basal lamina and/or the differential resistance of the connective tissue in various compartments of the lung. The liver metastases were further characterized by the development of a dense peripheral network of portal vessels and of peculiar fluid-filled spaces in the centers of the metastases. In agreement with previous work, destruction of elements of the adjacent tissue was hardly detected in the small metastases studied.