Extracts of Ascophyllum nodosum, Fucus serratus, F. vesiculosus and Pelvetia canaliculata contain inhibitorsof a-amylase, lipase and trypsin. The inhibitors were isolated and identified by 'H NMR spectroscopy as polyphenols which have apparent molecular weights in the range from 30000 to 100 000 daltons, as determined by ultra-filtration with Amicon membranes. These polyphenols account for the whole of the inhibitory activity in crude algal extracts. The compounds inhibit a-amylase and trypsin in an apparently non-competitive manner, when preincubated with the enzymes, and the inhibition is directly proportional to the concentration of the inhibitor. Starch protects a-amylase when added to the enzyme together with the inhibitors. Under this condition the effectiveness of the inhibitors is reduced ten-fold.