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What are endotoxins?

Written by [[CONTENT AUTHOR NAME]]Reviewed by [[SCIENTIFIC REVIEWER NAME]], [[REVIEWER CREDENTIALS]]Updated July 28, 2026Editorial policy

Endotoxins are lipopolysaccharide fragments from the outer membrane of Gram-negative bacteria. They are heat-stable, so they can persist after a process that has killed the organism, and they are screened for separately from sterility.

Because endotoxins survive conditions that destroy bacteria, a material can be sterile and still carry them. That is why endotoxin is its own test rather than something inferred from a sterility result.

Whether the parameter is appropriate depends on the material and the research context. It is a more significant question for a recombinant protein expressed in a bacterial system than for a short peptide assembled synthetically.

Where a lot's panel includes endotoxin, the result appears on that lot's certificate. Where it does not, no figure is shown — we do not report a parameter that was not tested.

How the test works, and why the units look unusual

Endotoxin is measured by the Limulus amebocyte lysate assay, which exploits a clotting cascade in horseshoe crab blood that is triggered by lipopolysaccharide with extraordinary sensitivity. Results are reported in endotoxin units — EU — per milligram or per millilitre. EU is a potency unit defined against a reference standard rather than a mass unit, which is why the figure is not expressed in nanograms.

The relevance of the number depends entirely on the experimental system. Cell culture work is the usual reason it is asked about, because endotoxin is a potent stimulus in many cell types at concentrations low enough to be invisible to any other test. In an assay of that kind, endotoxin contamination does not merely add noise — it can produce a coherent-looking result that is entirely an artefact.

If your work requires a specific endotoxin limit, check the parameter on the lot's certificate before ordering rather than after. Where a lot's panel does not include it, the honest answer is that the value is unknown, and an unknown value is not the same as a low one.