The conceptually and practically simple acute toxicity test provides information that is useful in the protection of aquatic life from pollution. Standardization increases the usefulness of routine acute tests by increasing their quality and comparability and allowing the study of various aspects of acute toxicity. For example, available data show that for certain materials some species of aquatic animals are at least 5,000 times more acutely sensitive than other species, whereas for other materials the range of acute sensitivity is less than a factor of 50. Various theoretical interpretations of the relationship of toxicity to time within a test have been proposed to improve the usefulness of the acute test, but such theories have not been supported by adequate pertinent data. The usefulness of routine acute toxicity tests can be increased, however, by basing the results not on one effect or another but on a combination of all severe adverse effects, such as death, immobilization, and loss of equilibrium.