Lysosomes degrade a wide range of macromolecules to yield monomer products which are exported out of the lysosome by a series of transporters. In addition, lysosomes perform a range of other functions which are cell or tissue specific. In order to gain insight into the tissue specific role of lysosomes, carrier-ampholyte two-dimensional electrophoresis (2-DE) was used in combination with N-terminal sequencing to identify the major proteins present in both the membrane and luminal space of placental lysosomes. From the 45 N-terminal peptide sequences generated, 14 luminal and five membrane proteins were identified while three other sequences were novel. The sequenced proteins were a mixture of lysosomal and non-lysosomal proteins. The lysosomal proteins consisted of gamma-interferon-inducible protein (IP-30), Saposin D, cathepsins B and D, beta-hexosaminidase, palmitoyl protein thioesterase, alpha-glucosidase, and LAMP-1. The non-lysosomal proteins were serum albumin, serotransferrin, haemoglobin gamma G chain, alpha-1-antitrypsin, placental lactogen, endoplasmin, peptide binding protein 74, p60 lymphocyte protein, p450 side chain cleavage enzyme and placental alkaline phosphatase. The 2-DE maps obtained in this study are the first to identify the major proteins in both the lumen and membrane of placental lysosomes through sequence analysis, and thus provide the basis upon which to build a complete 2-DE database of the lysosome. Furthermore, the identities of the proteins sequenced from the placental lysosomes suggest a role for lysosomes in the transport of nutrients across the trophoblastic layer.