Acute toxicity tests using mercuric ions (Hg2+) at the concentration of 10 and 50 mug L-1 were carried out using Escherichia coli as the organism test. Toxicity was evaluated by measuring the inhibition in the microbial respiration, at 37-degrees-C, within 60 min after the metal spike. It has been shown that mercuric ions are reduced to the much less toxic form of elemental mercury (Hg0) by the glucose, which is usually present in the microbial culture medium as a carbon source. Based on this evidence, inhibition in the microbial respiration caused by the presence of 5,000 mug L-1 of Hg0 were predicted to be similar to the effect caused by 10 mug L-1 of Hg2+ ions. This hypothesis has been proved to be true under the experimental conditions used in the bioassays.5397100