The LAL (Limulus Amebocyte Lysate) test is a widely used assay for detecting the presence of bacterial endotoxins, which are components of the outer membrane of Gram-negative bacteria. The test is based on the clotting reaction of the blood of the horseshoe crab (Limulus polyphemus) in the presence of endotoxins.

When LAL reagent, which contains amebocytes from horseshoe crab blood, is mixed with a sample suspected of containing endotoxins, a clotting reaction occurs if endotoxins are present. The extent of the reaction can be measured to quantify the amount of endotoxin in the sample.

The LAL test is commonly used in the pharmaceutical and medical device industries to ensure that products such as injectable drugs, implantable devices, and surgical instruments are free from endotoxin contamination, as endotoxins can cause fever, inflammation, and other adverse reactions in humans.