Surface pressure versus area isotherms have been coupled with fluorescence microscopy to compare the insertion preferences of a series of triblock copolymers of the form poly(ethylene oxide) - poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) into dipalmitoylphosphatidylcholine (DPPC) and dipalmitolylphosphatidylglyercol (DPPG) monolayers at the air-water interface at 30 degreesC. Previous monolayer experiments with one of the polymers of the series, Poloxamer 188 (P188), which is known to effectively seal electrically damaged cell membranes, show that P188 is able to insert into model monolayer systems of DPPC and DPPG at surface pressures close to or lower than its maximal spreading pressure(1) of 22 mN/m at 30 degreesC. To test whether the size of the hydrophobic PPO subunit regulates the insertion capabilities of the polymer into the lipid films, we have investigated the effect of sister poloxamers P108, P238, and P338 that have identical PPO/PEO weight percentages as compared to P188 but differ in overall molecular weight. While the smallest polymer investigated, P108, is able to insert into the lipid films at a surface pressure equal to its maximal spreading pressure, the larger polymers, P238 and P338, can insert only at pressures much lower than their respective maximal spreading pressures. However, all polymers investigated are "squeezed out" or eliminated from both monolayers at surface pressures significantly higher than those for insertion, results which mirror those obtained for P188. Although the bulkiness of the larger polymers limits their ability to insert into the lipid monolayer at pressures close to their maximal spreading pressures, their larger hydrophobic subunits seem to help them maintain their positions in the monolayer once inserted.