We report on interactions between a monomolecular film of 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) and bilayer vesicles of POPC suspended in the subphase of a Langmuir film balance. A large decrease in area of the monolayer held at a constant surface pressure pi of 30 mN m-1 was observed in the presence of either small or large unilamellar vesicles. By using [C-14]-POPC, we found that there was no appreciable lipid transfer from vesicle to monolayer or vice versa. The decrease in area of the monolayer held at pi = 30 mN m-1 is attributed to a localized reorganization of lipid in the film due to the transient collisional contacts between subphase vesicles and monolayer without significant transfer of lipid. POPC monolayers held at constant surface pressures pi of 15 and 20 mN m-1 did not show area decreases in the presence of vesicles. This indicated that the transient collisional contacts between vesicles and monolayer did not cause localized reorganization of lipid in the film when the monolayer was held at pi = 20 mN m-1. We also found that the size of the vesicles had little influence on the interactions between monolayer and vesicles. In contrast, the physicochemical nature of the Teflon surface strongly influences the behavior of the system.