A lethal dose (LD) is an indication of the lethality of a given substance or type of radiation. Because resistance varies from one individual to another, the 'lethal dose' represents a dose (usually recorded as dose per kilogram of subject body weight) at which a given percentage of subjects will die.

The most commonly-used lethality indicator is the LD50, a dose at which 50% of subjects will die.

Animal-based LD measurements are a commonly-used technique in drug research, although many researchers are now shifting away from such methods.

LD figures depend not only on the species of animal, but also on the mode of administration of the toxin; for instance, a toxin inhaled or injected into the bloodstream may require a much smaller dosage than if the same toxin is swallowed.