In 1927, J.W. Trevan attempted to find a way to estimate the relative poisoning potency of drugs and medicines used at that time. He developed the LD₅₀ test because the use of death as a "target" allows for comparisons between chemicals that poison the body in very different ways.

What does LD₅₀ mean?
LD stands for Lethal Dose. LD₅₀ is the amount of a material, given all at once, which causes the death of 50% (one half) of a group of test animals. The LD₅₀ is one way to measure the short-term poisoning potential (acute toxicity) of a material.

Toxicologists can use many kinds of animals but most often testing is done with rats and mice. It is usually expressed as the amount of chemical administered (e.g., milligrams) per 100 grams (or kilogram) of the body weight of the test animal. The LD₅₀ can be found for any route of entry or administration but dermal (applied to the skin) and oral (given by mouth) administration methods are the most common.

What does LC₅₀ mean?
LC stands for Lethal Concentration. LC values usually refer to the concentration of a chemical in air but in environmental studies it can also mean the concentration of a chemical in water.
For inhalation experiments, the concentration of the chemical in air that kills 50% of the test animals in a given time (usually four hours) is the LC₅₀ value.

Why study LC₅₀?
Chemicals can have a wide range of effects on our health. Depending on how the chemical will be used, many kinds of toxicity tests may be required. Since different chemicals cause different toxic effects, comparing the toxicity of one with another is hard. We could measure the amount of a chemical that causes kidney damage, for example, but not all chemicals will damage the kidney. We could say that nerve damage is observed when 10 grams of chemical A is administered, and kidney damage is observed when 10 grams of chemical B is administered. However, this information does not tell us if A or B is more toxic because we do not know which damage is more critical or harmful.

Therefore, to compare the toxic potency or intensity of different chemicals, researchers must measure the same effect. One way is to carry out lethality testing (the LC₅₀ tests) by measuring how much of a chemical is required to cause death. This type of test is also referred to as a "quantal" test because it is measures an effect that "occurs" or "does not occur."

Minimal inhibitory concentration (MIC) is the lowest concentration that prevents growth and the minimal lethal concentration (MLC) the lowest concentration that kills cells.

Prepare serial dilutions of the chemical in small sterile tubes, a cell well plate, or a Petrislide. A 2-fold dilution series is shown in the figure. Be sure to change pipette tips after each transfter and mix the contents of each well.

Add 2-5 ml of culture medium to each well and inoculate with the test organism; incubate.

Check the tubes or wells for growth. Here, growth is shown by a white dot in the bottom of the well. You can determine the growth/survivorship rate by plotting the number of cells against time to determine degree of inhibition.