1,31,4-@-~-Glucan 4-glucanohydrolases (EC 3.2.1.73) from Bacillus subtilis and barley (Hordeum vulgare) with identical substrate specificities but unrelated primary structures have been probed with (R,S)epoxyalkyl (-propyl, -butyl, -pentyl) 8-cellobiosides and with optically pure (3s)and (3R)-3,4-cellobiosides as active site-directed inhibitors. The optimal aglycon length for inactivation diff...