Beta-mannanase from Penicillium oxalicum SO efficiently hydrolyzed guar galactomannan to galacto-manno-oligosaccharides. Gel filtration estimated the molecular weight of the beta-mannanase as 35 000 and SDS-PAGE as 29 000. The optimum pH was around 5 while a stable pH was reached in the range of 3-6. Optimum temperature was around 60 degrees C at pH 5, while under 60 degrees C activity was stable. HPLC analysis detected oligosaccharides with degrees of polymerization (DP) of 2 to 7 and 2 to 6 released on hydrolysis of guar and locust bean gums, respectively; about 92% of the released sugars were oligosaccharides. In analysis of the sugar distribution on MALDI-TOF-MS, major products of DP 6 and 7 and DP 5 and 6 were confirmed in hydrolysates of guar gum and locust bean gum, respectively. One of the main oligosaccharides released from guar gum, with DP 7, had a high galactose content (Gal/Man = 0.76) and corresponded to a blockwise galactose-substituted mannan type in galactomannan.