Rat gastric surface cell membranes were prepared and the effect of taurocholic acid assessed by ESR spectroscopy using the 16-doxylstearic acid spin label. Taurocholic acid increased the polar part of the spectra, indicating an augmented amount of spin label molecules with a polar environment. Concomitantly, mobility of the spin label molecule was augmented. The effect of taurocholic acid was completely prevented by the previous addition of prostaglandin E2. This suggests a direct protective efficiency of prostaglandin E2 on rat gastric surface cell membranes without the metabolic participation of intact cells.