w. A. HEMMINGS The concept that proteins can enter cells whole is a difficult one. Yet the model situations whereby this process may be studied have been known and investigated for many years. Those situations arise through the specialization required to transfer immunoglobulins synthesized by the mother to the circulation of the fetus or newborn animal, that is in the transmission of passive immunity to the young. This always entails the protein crossing a continuous cellular barrier, in the placenta or fetal membranes, or in the intestinal epithelium. The concept is hard to accept in terms of cell biology because it is difficult to envisage a mechanism whereby proteins can pass in quantity a cell membrane which is regulating the entry of solutes. Brambell has put forward an hypothesis of such a mechanism, restating it in 'The transmission of passive immunity from mother to young' in 1970. In 1974, after his death, a symposium was held in Bangor largely on this subject, published as 'Maternofetal Transmission of Immunoglobulins' . Since then two further meetings have been held on gut transfer under the title of the present volume, which is composed of papers drawn from these two meetings.