The chemical forms and resistance to hydrolysis in vitro of raw and gelatinised starch from peas, maize, wheat and potatoes were measured. Raw granular starch proved very resistant to amylolysis. Only wheat starch was fully degraded after 24 hours' incubation with amylase (20 units/mg polysaccharide) at 37°C. In contrast, hydrolysis of freshly gelatinised starches was essentially complete within 1 h. To investigate the onset of resistance to hydrolysis after gelation, dispersions of amylose and amylopectin were stored at 20°C prior to amylolysis. Retrogradation of amylose was rapid, and the resulting material was highly resistant to amylolysis. In contrast, amylopectin underwent retrogradation more slowly and was almost completely degraded by amylase after incubation for 24 h. The onset of resistance to starch-hydrolysis in an amylose-rich food (cooked peas) was confirmed using a simulated digestion technique.