Abstract The problem of the toxic action of mixtures of many chemicals has been studied. Starting with the classification of Plackett and Hewlett, the types of mixtures for which toxicity can be predicted is discussed. A Mixture Toxicity Index (MTI) has been proposed as a quantification of the result of mixture toxicity experiments. Toxicity experiments with guppies have been conducted, using 6 mixtures of 3–50 chemicals. The toxicity of mixtures of simple similarly acting chemicals could be predicted by Concentration Addition. The concentrations of the separate chemicals in a mixture of 50 compounds giving 50% mortality, were 0.02 × LC 50.