Laboratory toxicity test results predict the responses of natural organisms with varying degrees of accuracy. The selection of test species and dilution water can greatly influence the applicability of the data in different situations. Other factors, such as nutrition, acclimation, and genetic selection, may influence the accuracy of predictions based on laboratory data. The relative importance of factors influencing the responses of organisms in the laboratory and in the field depends on whether the goal of the laboratory tests is to determine relative toxicity, to estimate safe levels, or to predict actual effects. When appropriate test parameters are chosen, the response of laboratory organisms is a reasonable index of the response of naturally occurring organisms.