We investigated the effects of three different bisimidoesters as homobifunctional cross-linking agents on the thermostabilization of penicillin G acylase (PGA) obtained from a mutant of Escherichia coli ATCC 11105. Cross-linkers were dimethyladipimidate (DMA), dimethylsuberimidate (DMS), and dimethyl-3,3'-dithiobispropionimidate (DTBP). The thermal inactivation mechanisms of the native and cross-linked PGA were both considered to obey first-order inactivation kinetics during prolonged heat treatment, forming fully active, susceptible transient stares. The efficacy of the cross-linkers on the thermostabilization of PGA was estimated to be DMA > DMS > DTBP. Optimal concentrations of DMA, DMS, and DTBP for cross-linking of PGA were found to be 0.5, 0.4, and 0.3% (w/v), respectively. The greatest enhancement of the thermostabilities was observed during DMA cross-linking, as a nearly 15-fold increase at temperatures above 50 degrees C. Cross-linking by DMA did not cause much change in the parameters V-m, K-m, and the optimal temperature values of PGA, but the activation energy of the enzyme was slightly decreased and k(cat) value slightly increased after cross-linking.