The increased release of heavy metals over the last century poses an unknown detriment to our ecosystem. Already. poisoning by toxic heavy metals has been documented in a number of species, including man. Estimation of the toxicological context of this release requires screening methods that rapidly process large numbers of samples with minimal cost, effort and ecological impact. We now describe a practical colorimetric kit to quantify mercuric ion in tissue, and demonstrate its application to screen fish. Advantageously, this test can easily be amended for field use and catch-and-release programs.