Standard measure of the toxicity of a material that will kill half of the sample population of a specific test animal in a specified period through exposure via ingestion, skin contact, or injection. LD50 is measured in micrograms (or milligrams) of the material per kilogram of the test-animal's body weight; lower the amount, more toxic the material. Used in comparison of toxicities, LD50 values cannot be directly extrapolated from one specie to the other or to humans. Also called median lethal dose. Written also as LD50.