(Toxicol; Abb: LD50) The median quantity of a substance that is fatal to 50% of test animals within a specified time period after ingestion or exposure. Syn: median lethal dose. Note: The x-ray LD-50/30 for man (i. e. , the dose of x-rays that within 30 days kills 50% of humans exposed) is about 400-450 roentgens