Transposon Tn 501-originated mercury resistance determinant (mer operon) located in plasmid pHP45 Omega-Hg was subcloned into plasmid pJF118EH, containing the tac promoter, and was subsequently transformed into Escherichia coli VJS632QA. The resulting recombinant strain E. coli PWS1 was able to tolerate Hg2+ concentration up to 80 mg/L and also exhibited higher specific mercury reduction activity than that of wild-type mercury-resistant strain Pseudomonas aeruginosa PU21 (Rip64). The effects of inducers (IPTG and Hg2+) on the mercury reduction activity of the PWS1. strain were identified. The mercury reduction kinetics were determined for the intact cells, permeabilized cells, crude cell extract, as well as purified mercuric reductase. The results show that the expression of mer operon in E. coli PWS1. was induced by IPTG and mercuric ions, while the addition of 0.4 mM IPTG resulted in a better enhancement on the enzyme activity than the addition of 5 mg/L Hg2+. Maximal mercury reduction activities occurred at initial Hg2+ concentration of 16 mg/L for intact cells and around 10-12 mg/L for the others. The activity of permeabilized cells was better than that of intact cells, suggesting that the transport of Hg2+ across the cell membrane appreciably retards the overall mercury reduction rate. The maximal specific activity of the purified enzyme was 0.41 mg Hg/mg total protein/sec, approximately 120-fold higher than that of the crude cell extract. The dependence of mercury reduction activity on substrate (Hg2+) concentrations can be described by substrate inhibition and enzyme deactivation models.