SummaryImmobilized penicillin acylase has been used for the deacylation of benzylpenicillin at 37°C in a continuous reactor consisting of four I liter stirred tanks connected in series. There was good agreement between the predicted and actual conversions obtained in each tank under steady-state conditions. The operational stability of the immobilized enzyme in the tanks depended on the pH and the rate of addition and concentration of alkali needed to neutralize the acid produced during the reaction.At pH 7 with the addition of 2M NaOH, the half-life for enzyme stability was greater than 400 hr in all tanks. This was over half the value for the immobilized enzyme when stored at 37°C and pH 7.