The frequent occurrence of xenobiotic chemicals in surface water is of concern to regulatory agencies and the public. The impact of these must be distinguished from impacts due to other factors such as low oxygen, habitat or dredging before an understanding of expected improvement from their reduction can be achieved. An ambient toxicity test is described, which is competitive in cost to G.C. analyses, that can be used to discuss if the chemicals present are causing toxicity. In field studies completed recently, the correlation between toxicity measured in ambient tests and the impact on biological communities was highly significant.