In toxicology, the LD50 of a particular substance is a measure of how much constitutes a lethal dose. In toxicological studies of substances, one test is to administer varying doses of the substance to populations of test animals; that dose administered which kills half the test population is referred to as the LD50, for "Lethal Dose, 50%".
The usual terms for expressing the LD50 are in units of mass of substance per mass of body mass, eg grams (of substance) per kilogram (of body mass). Stating it this way allows the relative toxicity of different substances to be compared, and allows one to scale for the different size of the animals exposed.
The choice of the 50% mark avoids the potential for ambiguity of making measurements in the extremes.
Unsurprisingly, animal welfare groups (particularly those influenced by the animal liberation movement) object to the studies needed to calculate this figure. This is particularly the case where the substance is not particularly toxic and a large quantity of the material is ingested by the animals over a long period, in some cases causing slow, painful deaths.
As concern grows for the welfare for animals, the test is administered less frequently, though the collection of data already obtained make it useful. Estimated LD50 numbers can be compared to those older numbers obtained more traditionally.
A comparable measurement is LCt50 which relates to lethal dose by inhalation, where C is concentration and t is time. It is usually expressed in terms of mg-min/m³. ICt50 is the dose which will cause incapacitation rather than death. These measures are commonly used to indicate the comparative efficacy of nerve agents.