(LD) the concentration of a substance required to kill the lab animal used for the test with a specific material.

a measure of the lethality or toxicity of an agent. The LD50 is the amount of a microorganism or a substance which will kill 50% of the subjects to which it is administered.

the amount of a toxic substance required to cause death of an organism under study in a given period of time.

A dose of ionizing radiation sufficient to cause death. Median lethal dose (MLD or LD50) is the dose required to kill, within a specified period of time, half the individuals in a large group of organisms similarly exposed. The LD50/60 for humans is about 4 Gy (400 rad).

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.