The breast stroma, in association with normal and neoplastic epithelium, is shown to contain cells of similar ultrastructure to the fine lymphatic vessels and sinusoids described in detail in the literature on other organs. As in other organs, the lumen of these structures is in continuity with the tissue spaces, allowing a free flow of lymph and its content, fluid and cells, from the tissues to the drainage system. The lymphatic system is, thus, open-ended-the endothelial cells of its finest branches having overlapping junctions that open passively on demand. There is, therefore, no need to postulate that cancer cells actively invade the surrounding tissues. In addition, the breast stroma contains endothelial cells in which the luminal side is not apposed to that of the same or another endothelial cell. These, like other endothelial cells, are attached to the connective tissue elements on one side only. It is suggested that they provide a series of potential spaces that can conduct fluid rapidly through the tissues, providing the epithelial cells with a copious supply of nutrients. This irrigation system is termed the lymphatic labyrinth, to distinguish it from the classical sinusoids and collecting vessels of the drainage system.