Heavy metal toxicity was studied by assaying six microbiological toxicity tests, both in solution and wastewater. Pseudomonas fluorescens and baker's yeast (Saccharomyces cerevisiae) were used; growth and respirometric determinations were performed. In addition, the Microtox test was employed as a reference method. The Microtox test is the most sensitive assay for detecting toxicity of zinc, copper, and mercury but not for cadmium, chromium, and nickel. Wastewater increases the sensitivity threshold (EC20) and EC50 values of the metals in most of the assays, which is correlated to the presence of organic and inorganic compounds that can reduce the bioavailability of the metals, leading to a general loss of sensitivity. All the above-mentioned assays are potentially useful in the detection of chemical toxicity of metals. However, each test shows different sensitivities to each metal, which is related to different sensitivities of the organisms used in the assays, as well as to other factors. Therefore, it would be advisable to use a battery of tests for biological evaluation of metal toxicity.