A sensitive but simple fluorimetric assay for intestinal enzymes capable of hydrolysing certain glycine-containing peptides is described. In the standard assay, 1 nanomole of glycine can be accurately estimated in the presence of a 2000-fold molar excess of unhydrolysed peptide. Initial rates of peptide hydrolysis can be measured on the density gradient fractions obtained by analytical subcellular fractionation of extracts from milligram quantities of human jejunal tissue. Differences in the subcellular localisation of activity against two peptides are illustrated. The implications of these findings in relation to the study of intestinal peptidase deficiency in disease states are discussed.