Deactivation of a lipase was investigated in a stirred vessel using two types of impellers, namely, six-bladed down-flow turbine and six-bladed straight turbine. The impeller speed was varied over the range of 16.67-33.33 rps with corresponding variation of the power consumption per unit volume from 1 to 70 kW/m(3). Effect of enzyme concentration on the deactivation, under otherwise identical conditions, was also investigated. The enzyme underwent negligible deactivation in the absence of air-water interface. In the presence of the interface and for lower enzyme concentration, it showed first order deactivation. The first order deactivation constant was well correlated with the power consumption per unit volume and the gas hold-up. The rate of enzyme deactivation was decreased by the addition of polyethylene glycol into the enzyme solution. Steady-state fluorescence studies were also done on the lipase during its deactivation. Based on this investigation, a strategy has been suggested for the improvements in the yield of enzymes in commercial production.