The transformation of DMPC vesicles containing myristic acid to larger structures was examined by nuclear magnetic resonance techniques. The process was found to be characterized by a rate which is approximately first order in vesicle concentration. Rates, however, show a dramatic increase with increasing myristic acid content. The fatty acid is characterized as being active under conditions in which it is neutral and dispersed in the bulk bilayer phase. In addition a 1:1 correlation was noted between leakage of contents and vesicle transformation. On the basis of these observations a mechanism is postulated involving stabilization of leaks in bilayer membranes by myristic acid, and the resultant formation of activated sites for vesicle transformation. The relation to fusion in natural systems is discussed.