Detection of this activity or residual toxicity remaining after biodegradation underscores the need to test for toxicity changes during biodegradation studies. The occurrence of contaminants in mixtures like crude oil is an important problem because the removal or degradation of one component can be inhibited by other compounds or by-products in the mixture, and because different conditions may be required to treat different compounds within the mixture. Bioassys provide important information for the assessment of pollutant effects. In contrast to chemical analyses, they detect effects of multiple contaminants, by-products and metabolites. A large number of bioassays have been applied for the characterization of contaminated soils and for the evaluation of bioremediation efficacy [35-37].