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A recombinant plasmid containing the complete lacZ gene downstream of the T7 promoter was used to transform Escherichia coli containing another plasmid which had the T7 RNA polymerase gene under the control of heat inducible λP L promoter. This recombinant E. coli containing the two plasmids was studied in order to enhance β-galactosidase expression. The heat shock time which effectively regulates the T7 RNA polymerase was optimized and best expression of β-galactosidase was obtained with 2 min heat shock. Substrate feeding increased the duration of log phase and allowed induction at a higher cell density without affecting the specific activity. A high cell density (7 g l -1 ) and high specific activity (~20 000 U) were achieved which effectively increased the product concentration 18-fold.