Lipase from Geotrichum candidum (ATCC34614) is a glycerol ester hydrolase which has a molecular weight of 55,000 with about 7% carbohydrate, displaying a high affinity for triolein. The enzyme was crystallized from more than 2% protein solution without using any salt or organic solvent. The crystals were cross-linked by soaking in 0.37% glutaraldehyde solution (0.1 M acetate buffer solution, pH 5.6). The structure was determined by X-ray diffraction using the isomorphous replacement technique. Two heavy-atom derivatives [K2PtCl4 and UO2(CH3COO)2] were obtained by the soaking method. The electron density map calculated at 5 A resolution clearly showed the molecular boundary. A balsa wood model was made on the basis of the 6 A electron density map. The molecular has an ellipsoidal shape with dimensions of 70 A X 50 A X 50 A. Several columns of density corresponding to alpha-helix and a few clefts were found in the molecule. The active site is presumably located in the vicinity of one of the Pt sites in the Pt-derivative crystal, judging from the inactivation of the enzyme by K2PtCl4.