Abstract: A sample of an aqueous liquid, e.g. an aqueous parenterally-administered drug, is tested for determining the amount of any endotoxin contained therein by admixing a sample of the liquid with lysate obtained by water lysing washed amebocyte cells from the blood of the horeseshoe crab (Limulus polyphemus), said lysate being buffered at a pH between about 6.5 and about 6.8 and being at a concentration such that 1 ml thereof is reactive with 0.1 ml of an endotoxin solution containing 1 ng per ml of endotoxin to develop upon incubation for 60 minutes at 37.degree. C turbidity exhibiting absorbancy between about 0.07 and about 0.