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Assemblages of marine benthic invertebrates have been shown to differ in areas with or without contamination by heavy metals. In this study, soft sediment assemblages were exposed to sediments and water treated with copper (in the form of Cu EDTA) in a laboratory experiment to test the hypothesis that copper could create differences in infaunal assemblages. The patterns of survival of all taxa were monitored and compared to the controls. Copper was found to reduce the abundance of most taxa, but the size of the response varied between taxa. Crustaceans were particularly sensitive to both the laboratory conditions and copper and were the most severely affected. Polychaetes were more resilient, surviving well in the controls and being reduced in abundance in polluted treatments, but with some families persisting in even the severely polluted treatment until the end of the experiment. Bivalves and gastropods were also reduced in number in the polluted treatments and unaffected in the controls. Oligochaetes and nemerteans did not respond in any particular pattern to any treatment or control. The results are discussed in relation to extrapolating results from ecotoxicological studies to predictions of ecosystem response to heavy metal pollution and pollution in general. It is concluded that laboratory ecotoxicological studies provide excellent evidence for potential effects. If used in collaboration with surveys and manipulative experiments in the field they can demonstrate causal links between pollutants and environmental impacts.