We have compared the properties of two N-acyl derivatives of dilauryl phosphatidylethanolamine on lipid polymorphism, vesicle leakage and Sendai virus fusion. The derivatives contained either an N-lauroyl group (NLPE) or an N-acetyl group (NAcPE). Only the NAcPE markedly affected the bilayer to hexagonal transition temperature of dielaidoyl phosphatidylethanolamine, shifting it to higher values. In contrast the NLPE slightly lowered this phase transition temperature. The two lipids also have opposite effects on leakage from small unilamellar vesicles of egg phosphatidylcholine. The NLPE inhibits leakage, while the NAcPE promotes it. This vesicle stabilizing effect of NLPE against leakage is not manifested in alterations of rates or extents of Sendai virus fusion to liposomes of egg phosphatidylethanolamine plus 2% ganglioside GD1a. The NLPE has no effect, while the NAcPE reduces the observed fusion, at least in part as a consequence of a reduction in the final extent of fusion. These results demonstrate that the bilayer stabilizing effects of NLPE do not result in a lower rate of viral fusion. Furthermore, these bilayer stabilizing effects against leakage are not solely a function of the lipid headgroup but also require a structure with three long acyl chains. The reduced leakage is not related to a loss in monolayer curvature strain.