Lymph from the mammary gland passes through at least three lymph nodes. These are usually the mammary, iliofemoral and medial iliac, although the subiliac, internal inguinal and sacral lymph nodes may also be involved. The iliofemoral, a large, flattened node in the dorsolateral wall of the pelvis, receives lymph from subiliac and popliteal as well as mammary nodes. Afferent lymphatic vessels either enter the subcapsular sinus on the dorsal surface of this node, or penetrate more deeply into the node. Lymphatics carrying mammary lymph enter more caudally than those from the popliteal or subiliac lymph nodes. Evans blue dye, injected into the mammary gland, showed a characteristic distribution within the mammary and iliofemoral nodes. Within the iliofemoral nodes it was present mainly in the caudal part. However, when China ink was used, carbon particles were distributed throughout most of the ipsilateral mammary nodes. In the iliofemoral node the carbon was distributed more widely than the Evans blue; it was found, mostly within macrophages, over the whole length of the node. A similar distribution of carbon was seen when it entered from the popliteal node. It appears likely that, although lymph from the mammary gland shows some preference for the caudal part of the iliofemoral node, constituents of this lymph may mix within the node with lymph from other primary nodes.