The LD50 is the dose that kills half (50%) of the animals tested (LD ="lethal dose"). The animals are usually rats or mice, although rabbits, guinea pigs, hamsters, and so on are sometimes used. It is most often expressed as mg of product per Kg of animal body weight or as a concentration of ppm.

It is also expressed on a logarithmic scale of infectious units. Experiments report that, for example, concentrations for transmission of BSE to mice are 107, but for mouse TSE transmission to mice of different strains is 104. This means it requires a concentration 1,000 times higher for BSE infection than mouse to mouse TSE infection.