Breast tissues from 59 patients were immunohistologically studied for transferrin and transferrin receptors. None of the 8 normal breasts or 22 benign mastopathies, and only 1 of 7 fibroadenomas were reactive for transferrin, but 16 (72.7%) of 22 carcinomas were positive and parallel sections showed histopathologically that the rective areas were confined to tumour-cell membranes. That breast-tumour transferrin is probably receptor-bound in vivo was indicatd by its complete chemical removal in vitro and return to the same cells following reincubation with dilute solutions of transferrin. Similar bindig is found on certain normal or transformed cells and at the maternofetal interface of human placentae. It is suggested that this oncofetal receptor preferentially binds apotransferrin and may offer a novel approach to more specific therapy of certain breast cancers.