A number of medium additives such as Pluronic F68, methylcellulose, and serum have been shown to decrease the adhesion of animal cells to air bubbles, thus reducing cell damage by the bubbles at rupture. The effect may be associated with the interactions between the additives and the cells. One possible mechanism is that the additives adsorb to the cell membrane through a hydrophobic interaction, resulting in decreased hydrophobicity of the cell surface. This consequently reduces cell adhesion to gas bubbles. To rest this hypothesis, we measured the hydrophobicity (adhesion to a hydrocarbon) of two insect cell lines in the presence of medium additives including Pluronic F68, methylcellulose, polyethylene glycol (PEG), and fetal bovine serum. All these additives except PEG caused substantial reduction in cell surface hydrophobicity which was consistent with their effect of decreasing cell adhesion to gas bubbles. In addition, significant adsorption was detected for the nonionic surfactants Pluronic and PEG to the insect cells. The findings are very helpful for elucidating the mechanisms of animal cell protection by surface-active chemicals.