The inhibitory specificity and structural features of two amylase inhibitor preparations (designated BAYe 4609 and BAYg 5421) from Actinoplanes sp, have been studied . These inhibitors are carbohydrates, consisting mainly of glucose, but also having a nitrogen containing moiety present (D. SCHMIDT ct al., Naturwiss., 64, 535, 1977) . BAYe 4609 inhibits α-amylase rather more strongly than it inhibits glucoamylase, although the inhibitory specificity towards different α-amylases differs in magnitude. Further studies on BAYg 5421 were performed; acid hydrolysis of the inhibitor decreased the inhibitory activity towards glucoamylase, but increased more than 100 times towards α-glucosidase. It was also discussed on the pH effect of the inhibitory activity .