LD₅₀ is the dose of a chemical that is lethal to 50 percent of a test population. It is therefore a measure of a particular median response which, in this case, is death. The term is most frequently used to characterize the response of animals such as rats and mice in acute toxicity tests. The term is generally not used in connection with aquatic or inhalation toxicity tests. It is difficult, if not impossible, to determine the dosage of an animal in such tests; results are most commonly represented in terms of lethal concentrations (LC), which refer to the concentration of the substance in the air or water surrounding an animal.

In LD testing, dosages are generally administered by means of injection, food, water, or forced feeding. Injections are used when an animal is to receive only one or a few dosages. Greater numbers of injections would disturb the...