Lactating cow and rat tissues were examined for gamma-glutamyl transpeptidase. The specificity of the enzyme for various amino acid acceptors was determined for mammary tissue. The relative specific activity of gamma-glutamyl transpeptidase was high in cow mammary tissue and lower in rat mammary tissue in relation to the activity in kidney of the respective mammals. However, activity per gram wet weight of mammary tissues appeared equivalent. gamma-Glutamyl transpeptidase activity towards specific amino acids agreed with the percent extractions of these amino acids by the mammary gland of the lactating cow in vivo. We believe this is the first evidence of a possible system of amino acid transport by mammary tissues.