Responses to copper and sodium pentachlorophenate at various nutrient concentrations, up to COD=1500 mg/l, were compared in different types of microbial cultures : Escherichia coli and activated sludge. In the E coli. culture, effective concentration at 50% inhibition varies from 0.745 mg/l to 11.56 mg/l (copper) and from 1.963 mg/l to 8.163 mg/l (sodium pentachlorophenate) when COD concentrations change from 30 mg/l to 1500 mg/l, respectively. For the activated sludge, however, EC50 remains stable, indicating toxicity is independent of nutrient status. This phenomenon is mainly related to the species composition which determines the correlation coefficient. Results in this study indicate that the influence of the type of microbial cultures is a crucial factor in determining microbial growth dynamics.