The Malacca river runs through the Malacca UNESCO heritage site where a number of historical buildings are located. The river itself runs through several industrial sites that increase the chances of the water being polluted. Water pollution including heavy metals, in the long run, can damage the reputation of the site. Hence monitoring of the water quality needs to be done periodically. As the cost of instrumental monitoring is costly, biomonitoring using enzyme is being intensely developed worldwide. In this study, a rapid inhibitive enzyme assay using the molybdenum-reducing enzyme from the bacterium Serratia sp. strain DRY6 sensitive to the heavy metals mercury, copper, silver, and chromium was developed as a method for a rapid monitoring of heavy metals. The IC¬50 values for mercury, copper, silver and chromium were 0.268, 0.352, 0.393 and 0.499 mg L-1, while the LOD values were 0.166, 0.071, 0.033 and 0.064 mg L-1, respectively. The IC50 values for these heavy metals are comparable and in several cases, more sensitive than established assays. Water samples from various locations in the Melaka river were tested for the presence of heavy metals using the developed assay. Enzyme activity was found to be inhibited in one sampling location, but the concentration of metal ions on the site was found to be below the Maximum Permissible Limit according to Malaysian Environmental Quality standard. The assay for heavy metals can be completed in less than 10 minutes and can be carried out at ambient temperature. The assay is rapid and simple and can be used as a first screening method or even near real-time method for routine monitoring of heavy metals.