The effect of a proteinase inhibitor extracted from potatoes (POT II) which increases CCK release, on food intake was examined in 11 lean subjects. They received 1.5 g POT II in a high-protein soup vehicle (70 kcal), the soup vehicle alone, or a no-soup control five minutes before being presented with a lunchtime test meal, according to a double-blind, within-subjects design. Consuming the soup alone led to a nonsignificant 3% reduction in energy intake. The addition of 1.5 g POT II to the soup significantly reduced energy intake by a further 17.5%. Premeal ratings of motivation to eat and food preferences did not predict the reduction in energy intake by the proteinase inhibitor. These findings suggest that endogenous CCK may be important in the control of food intake and that proteinase inhibition may have therapeutic potential for reducing food intake.