Lysosome membranes from rabbit polymorphonuclear leukocytes free from other cellular contaminants and with minimal enzymatic adsorption have been obtained Chemical constituents of these membranes were in the same proportion as in the plasma cell membranes except for the smaller number of polypeptides. Molecular weights of the 12 lysosome membrane polypeptides, ranged from 9,200 to 480,000 as estimated by acrylamide gel electrophoresis, seven of which gave a positive periodic acid-Schiff (PAS) staining. Lysosome membranes were solubilized by using formic acid as a disrupting agent and 8 M urea for maintaining proteins in solution during the purification work. Proteins behaved nicely when applied to gel filtration or isoelectric focusing and three of them (mol. wts. 9,200, 37,800, 145,000) were isolated and chemically characterized. Carbohydrate content of the isolated proteins, was higher than in the whole membrane, at expenses of neutral sugars and methyl pentoses but with smaller amount of sialic acid. Amino acid composition of 9,200 MW protein was rich in arginine and non-polar aminoacids and that of the two others was rich in glutamic acid and glycine. This procedure represents a good approach to the study lysosome membrane proteins, from which a better understanding of the phagocytosis process could be obtained.