1. A pure lipid acyl-hydrolase was prepared from potato tubers by acetone precipitation, Sephadex G-100 and DEAE-Sephadex A-50 column chromatography, and by electrofocusing. 2. The purified enzyme was an acidic protein of pI 5.0 and molecular weight of about 70 000. Km values were 0.38 mM for monogalactosyldiacylglycerol and 1.7 mM for phosphatidylcholine. 3. The hydrolytic activity of the enzyme on different substrates was determined. The relative rates were acylsterylglucoside greater than monogalactosyldiacylglycerol greater than monogalactosylmonoacylglycerol greater than digalactosyldiacylglycerol greater than diagalactosylmonoacylglycerol, while the rates for phospholipids were lysophosphatidylcholine greater than phosphatidylcholine greater than lysophosphatidylethanolamine greater than phosphatidylethanolamine. 4. Analyses of enzymatic hydrolysis products suggested that a single enzyme had both galactolipase and phospholipase activities, and for the phospholipids it showed activities similar to phospholipase B and glycerylphosphorylcholine diesterase. 5. A competitive relation was found between monogalactosyldiacylglycerol and phosphatidylcholine as substrates of the enzyme, indicating that the active sites for both substrates may be the same. 6. It was suggested that histidine and probably serine residues were important to the enzymic activity, and that a tyrosine residue might be involved in the activity as an accessory component.