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In toxicology, the median lethal dose, LD < sub > 50 </ sub > ( abbreviation for “ lethal dose, 50 %”), LC < sub > 50 </ sub > ( lethal concentration, 50 %) or LCt < sub > 50 </ sub > ( lethal concentration & time ) of a toxin, radiation, or pathogen is the dose required to kill half the members of a tested population after a specified test duration.

LD < sub > 50 </ sub > figures are frequently used as a general indicator of a substance's acute toxicity.

The test was created by J. W.

Trevan in 1927.

The term semilethal dose is occasionally used with the same meaning, in particular in translations from non-English-language texts, but can also refer to a sublethal dose ; because of this ambiguity, it is usually avoided.

LD < sub > 50 </ sub > is usually determined by tests on animals such as laboratory mice.