The end-products of lysosomal digestion of biopolymers (amino acids, monosaccarides, nucleosides, etc.) are known to exit the lysosome through the agency of specific membrane carrier proteins. It is currently believed that in humans, defects in these carrier proteins can result in a number of inherited storage disorders which are characterized by intralysosomal entrapment of excessive amounts of specific metabolites such as cystine or sialic acid. Unlike the plasma membrane carrier proteins, many of which have been isolated and/or sequenced, nothing is known concerning the structure of the lysosomal membrane carriers. Among the approaches which may be followed to accomplish this characterization is their purification from isolated lysosomal membrane vesicles. However, lack of an appropriate in vitro assay for carrier function has served as a serious obstacle to such attempts. We are currently investigating such an assay system through the use of reconstituted artificial lipid membrane vesicle preparations (proteolipo- somes). Uptake of specific radio- labeled ligands such as cystine or sialic acid would presumably signify retention of functional carrier activity by the lysosomal membrane fractions and assist in monitoring their further purification.