The mechanism of tumor localization of gallium-67 (67Ga) is not known with certainty, although much information has been derived regarding the biodistribution and subcellular fate of 67Ga in a variety of tumors and other tissues from experimental animals. After intravenous administration, 67Ga is bound to transferrin in the blood, and distributed to liver, lacrimal glands, salivary glands, and soft tissue tumors. Within the cells of the liver and tumors, gallium is found in lysosomes, and rough endoplasmic reticulum. Within these organelles, 67Ga is bound to a variety of macromolecules, including transferrin, ferritin, and a 45,000 molecular weight glycoprotein. Recent studies of tumor cells growing in tissue culture suggest an important role for transferrin in 67Ga tumor uptake. This uptake is mediated by a transferrin specific cellular receptor.