L.D. 50, or "lethal dose 50," is a standard measurement used in toxicology to determine the dose of a substance that is lethal to 50% of a test population. This value helps researchers assess the toxicity of chemicals, drugs, and other substances, providing critical information for safety evaluations and regulatory decisions. The L.D. 50 value is typically expressed in milligrams of substance per kilogram of body weight (mg/kg). It is commonly derived from animal studies, often using rats or mice, and serves as a benchmark for comparing the toxicity of various compounds, aiding in the development of safer products.