The effects of resistant starch on human metabolism are considered to be beneficial because they decrease postprandial glycaemia following ingestion of starch-rich foods. Using type II and IV resistant starches, we found a direct competitive inhibitory effect on porcine pancreatic a-amylase action above 0.25 g/100 g affecting catalytic efficiency when wheat and maize starches were used as substrates. Because resistant starches do not gelatinise and no modifications of gelatinisation parameters were observed, we concluded that the inhibitory action of these types of polymer seem to be a consequence of impaired diffusion of enzyme, substrate or products because of an increase in branches in the two types of resistant starches used. Thus, the effects of type II and type IV resistant starches in lowering postprandial glycaemia not only involve modification of gut physiology, but also include direct inhibition of the first stage in the biochemical degradation of starch.