LD50 tests on rat and mouse are commonly used to express the relative hazard associated with the acute toxicity of new and existing substances. These tests are expensive, time consuming and actively fought by Animal Rightists. Consequently, there is a need to find alternative methods. If the design of QSAR models can be used as surrogate, the search for interspecies correlations also represents a valuable alternative to the classical mammalian laboratory tests. This study investigates quantitatively the interspecies relationships of the acute toxicity of several hundreds of organic and inorganic chemicals to rat, mouse, Vibrio fischeri (Microtox), and Daphnia magna accounting for the experimental conditions of the different tests. For the first time, attempts have been made to test the usefulness of nonlinear methods for deriving interspecies toxicity correlations.