Uniform, unilamellar vesicles have been prepared by the pH-modification technique. The initial sizes of the vesicles were from 200 to 700 nm and were measured to within 1-3% by photo correlation spectroscopy. Vesicles were made of the dioleoyl esters of phosphatidic acid, phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine, the diphytanyl ethers of phosphatidylglycerol, Escherichia coli lipids, and lac permease reconstituted into E. coli lipids. The vesicle suspensions were prepared and then diluted with electrolyte (KCl) and/or nonelectrolyte (sucrose, trehalose, pentaerythritol) impermeants. The amplitude of the swelling is linearly proportional to the osmotic pressure difference across the bilayer. We have determined the elastic modulus, the elastic limit (percent surface expansion at bursting), and the transbilayer pressure difference at bursting for each of these vesicles at constant osmolarity but at different ionic strengths. We find that the elastic properties of the bilayer vary by a factor of 10 in electrolyte media as compared to isosmolal nonelectrolyte media and that this variation appears to be related to both the charge density at the surface and the ionic strength of the media. Anionic lipid vesicles in 150 mM KCl have a significantly higher modulus (50 X 10(7) dyn/cm2) and transbilayer pressure difference (40 mosM) at bursting with a small capacity to stretch (3-4% surface expansion) compared to the same vesicles suspended in nonelectrolyte impermeants. The latter vesicles undergo a significant surface expansion (8-10%), display a low modulus (3 X 10(7) dyn/cm2), and burst at 3-4 mosM bilayer pressure difference. Vesicles suspended in media of constant osmolarity at various ionic strengths display properties with proportional values.(ABSTRACT TRUNCATED AT 250 WORDS)