The pH-dependence of the reaction kinetics of lactase (b-galactosidase) [9031-11-2] from A. oryzae in different reaction media is presented in terms of a 2-parameter model. The lactase from A. oryzae seems to have replaced the A. niger lactase on the market owing to a better activity/price ratio and may be utilized for lactose hydrolysis in acid as well as in neutral milk products. Its pH optimum is .apprx.4.5. However, in the neutral pH range its activity in whey (pH 6.5) fell to only 30% of its expected activity in a pure lactose soln. at the same pH. The whey effect was the same for both sol. and immobilized lactase. The 2-parameter kinetic model, which included a term for competitive product inhibition, gave excellent agreement with exptl. data and may thus be useful for the prediction of reactor performance with this enzyme. [on SciFinder (R)]