Indirect colorimetric analyses of released reducing groups, as an indicator of induced beta-1,3-glucanase activities, tend to be the assay methods of choice for the characterization of plant endo- and exo-beta-1,3-glucanase, However, interfering low molecular weight reducing sugars found in unprocessed plant extracts often mask in vitro estimations of P-ld-glucanase activity. The enzyme-catalysed hydrolysis of an optically active beta-1,3-glucan polymer was monitored polarimetrically by measuring changes in the rotation of plane-polarized light of the reaction assay medium. A direct, non-radiochemical assay that detects changes in the optical rotation of released (1->3) beta-D-oligoglucosides with low degrees of polymerisation (<<25) has been found to be highly reproducible, rapid (total analysis time 15 min), sensitive (detection limit 0.007 unit/mL glucanase), selective and non-destructive. This assay has been used to detect a salicylate-induced cell wall-bound tomato beta-1,3-glucanase, which is a component of a pathogenesis-related response in plants. Copyright (C) 2000 John Wiley & Sons, Ltd.