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a dose of any chemical or physical agent that brings about the death of a living organism.
Lethal doses are determined in animal experiments for various pharmacological agents, pesticides, and ionizing radiation. Lethal doses vary both with the properties of the agents and with the age, condition, and individual sensitivity of the organism. Therefore, lethal doses must be determined experimentally on a fairly representative number of animals. A distinction is made between minimum lethal dose, which is fatal only to the most sensitive individuals of a species (no more than 10 percent of the experimental animals); median lethal dose, which is fatal to half (LD50) the animals; and absolute lethal dose, which is the smallest amount that will cause the death of all of the experimental animals (LD100).