Direct activity determination by a flow-through microcalorimetry in the enzyme thermistor system was employed for a fast comparison of (poly)acrylamide gel-entrapped penicillin G acylase preparations. Composition of the pre-polymerization cocktail and both the storage and operational stabilities of optimal gel-entrapped enzyme preparations isolated from the Escherichia coli industrial strain were optimized by this method. The validity of the results was corroborated by spectrophotometric measurements.