We have examined the effect of poly(ethylene glycol) (PEG) on stable large unilamellar vesicles formed by a rapid extrusion technique and composed of pure synthetic phosphatidylcholines. The lipid systems studied were the saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and the monounsaturated 1,2-dioleoyl-sn-glycerol-3-phosphocholine (DOPC). PEG at all concentrations (3.8-40 wt %) induced lipid mixing between large vesicles composed of these phosphatidylcholines. Extensive leakage of internal contents also occurred at high PEG concentrations. However, in contrast to our previous report [Parente, R. A., & Lentz, B. R. (1986) Biochemistry 25, 6678], we could detect no mixing of internal contents indicative of fusion. This discrepancy is due to environmental factors that affect the behavior of 8-amino-naphthalene-1,3,6-trisulfonic acid (ANTS), the fluorophore used in the assay for contents mixing and leakage [McIntyre, Parks, Massenburg, & Lentz (1991) (submitted)]. In agreement with the results of the fusion assay, quasielastic light-scattering measurements revealed no increase in vesicle size following treatment with PEG. These results emphasize the importance of using assays for both membrane mixing and contents mixing to demonstrate fusion, since significant lipid mixing occurred in the absence of fusion. We conclude that large vesicles composed of pure phosphatidylcholine do not fuse in the presence of even high concentrations of PEG. However, DOPC vesicles containing a small amount of an amphipathic "impurity" have been shown to fuse in the presence of PEG at 23 degrees C. These results are discussed in terms of their implications for the mechanism of PEG-induced membrane fusion.