The effect of pHs between 2.0 and 10.0 on the inactivation kinetics of penicillin G acylase (PGA) obtained from a mutant of Escherichia coli ATCC 11105 and the stabilization of enzyme against pH by chemical cross-linking with dimethyladipimidate (DMA) were studied. The inactivation mechanisms of native and DMA cross-linked PGA both appeared to obey first-order inactivation kinetics during prolonged incubation of enzyme solutions at 40 degrees C and at different pH values. The lowest inactivation rate constants were obtained with both native and DMA cross-linked PGA at pH 7.0. Inactivation rate constants decreased with an increase in pH from 2.0 to 7.0 and then increased again at pH values above 7.0. Chemical cross-linking of PGA by DMA resulted in the stabilization of enzyme against pH. Highest enhancement of pH stabilization (nearly four-fold) was obtained at pH 7.0 and 8.0.