The influence of phosphatidylcholine liposomes on the tryptic digestion of insulin was studied to obtain basic information on the interaction between liposomes and peptides or proteins. Protection of insulin from tryptic digestion by liposomes depended on the encapsulation efficiency and the method of preparation of liposomes. REV (reverse phase evaporation vesicles) were most effective for the protection of insulin from tryptic digestion. Tryptic digestion of insulin was accelerated by negatively-charged liposomes (containing 10% phosphatidylserine, phosphatidylinositol, or phosphatidic acid) or by neutral empty liposomes, this digestion being enhanced by progressively smaller liposome size (from 2 to 0.1 microns) in the case of neutral empty liposomes. These results suggest that this enhancement had occurred on the surface of the empty neutral or negatively-charged liposomes. Positively-charged empty liposomes (containing 10% of stearylamine) weakly suppressed the tryptic digestion of insulin. The size of the positively-charged liposomes increased after the addition of insulin, this size increase suggesting that these liposomes were fused by insulin. Presumably, insulin was protected by the lipids around the insulin molecule after the insulin-induced fusion or aggregation of positively-charged liposomes.