In the last 20 years, the mechanisms of protein absorption have been studied with increasing intensity, and a large book could be written on this subject alone. In spite of this volume of work, it has recently become clear that our knowledge of protein absorption is still far from complete, since until very recently nearly all investigations have been concerned with the intestinal transport of amino acids. Evidence for the existence of a second important mode of protein absorption-mucosal uptake of small peptides with cellular hydrolysis-is now extremely strong, and the importance of this mode of absorption is maintaining nutrition in cases of intestinal transport defects for amino acids has recently been demonstrated. Yet the details of mucosal oligopeptide transport have hardly begun to be explored. It is therefore difficult to give a balanced account of protein absorption at the present time. This account is selective, most attention being paid to growing points in the field. Several reviews covering the details of amino-acid absorption are available (Wilson