We investigated the effect of two nonlamellar-prone lipids, phosphatidylethanolamine (PE) and dioleoylglycerol (DOG), on the efficiency of protein encapsulation in liposomes. When the phosphatidylcholine (PC) matrix was replaced with PE or DOG during liposome formulation, the amounts of glutathione S-transferase and bovine serum albumin entrapped in the vesicles increased with increasing PE or DOG concentration. The presence of PE and DOG synergistically affected protein entrapment. These results suggest that protein encapsulation can be enhanced by the presence of nonlamellar lipids and/or lipid-induced membrane properties.