A specific carboxypeptidase that converts enkephalin precursors into enkephalin in adrenal chromaffin granules has been purified and characterized. In the adrenal this enzyme, designated enkephalin convertase, is uniquely localized to the chromaffin granules, which contain enkephalin and precursor peptides. Enkephalin convertase is markedly stimulated by CoCl2 and inhibited by EDTA or 1,10-phenanthroline, unlike the lysosomal carboxypeptidase. The purified enzyme has a high affinity for the hexapeptides [Met5]- and [Leu5]enkephalin-Arg6 (51 and 83 microM, respectively) and a somewhat lower affinity for the hexapeptides [Met5]- and [Leu5]enkephalin-Lys6 (195 and 174 microM). Brain enkephalin convertase shows 10-fold regional variations, unlike other carboxypeptidases, which are uniformly distributed. Enkephalin convertase appears to be associated selectively and physiologically with biosynthesis of the enkephalins.