The stability of neutral beta-mannanase from Bacillus licheniformis during operation and storage was investigated. The enzyme activity decreased by 70% with a hydrolysate of glucomannan at 20 g l(-1) over 30 min at 25degreesC. In an enzymatic membrane reactor operated at 50degreesC after 24 h, the loss of enzyme activities were 23% and 9% in the absence/presence of the substrate. The residual activities of the enzyme were 21% and 90%, respectively, when stored in 30% (v/v) glycerol solution and in solid state at 4degreesC after one year.