The protective effects of polymer additives on insect cells against fluid mechanical damage was investigated in a falling film-flow device. The falling liquid film creates rapidly moving air-liquid interfaces and high fluid shear stress, mimicking the characteristics of a bursting bubble in aerated cell culture. The additives tested included a group of surface-active polymers, (i.e., Pluronic F68, poly(ethylene glycol)s, and Tween 80) and a group of viscosity-enhancing polymers (i.e., dextrans, methylcellulose, and (carboxymethyl)cellulose). We found that methylcellulose, which was previously considered a viscosity-enhancing polymer, actually had significant surface-active properties. All of the surface-active polymers exhibited significant protective effects, with Pluronic F68 and the higher molecular weight poly(ethylene glycol), PEG 20M, providing the best protection, In contrast, the viscosity-enhancing polymers, with the exception of methylcellulose, showed little or no protection for insect cells in the film flow. All of the protective polymers had surface-active properties, even though some of them did not change the surface tension in the actual insect cell medium. There was no correlation between the protective effect and the changes in liquid viscosity and surface tension due to the polymer additives. The level of protection was shown to be dependent upon the type of polymer, its concentration in the culture medium, and the polymer molecular weight. We concluded that the mechanism of protection of these surface-active polymers was through interaction of the polymer molecules with the cell plasma membranes : a fast-acting biological mechanism.