“…Previous work in this laboratory has shown that the isolated perfused guinea-pig mammary gland absorbs essential amino acids from the perfusate in amounts related to the output of these amino acids in milk protein (Davis & Mepham, 1974). We have suggested that essential amino acids belong to one of two categories, group 1, consisting of methionine, tyrosine, phenylalanine, histidine and tryptophan, the uptakes of which agree very closely with their outputs in milk protein, and group 2, consisting of threonine, valine, isoleucine, leucine, lysine and arginine, which are absorbed in excess of their requirement for protein synthesis (Davis & Mepham, 1976;Mepham, Peters & Davis, 19766;Peters, Alexandrov & Mepham, 1979).…”