the presence of endotoxin in the outer membrane of cell wall of a gram - negative bacteria is used to determine bacterial contamination in a biological product . the amount of endotoxin present in a biological product is accurately measured without influence of a limulus reaction - activating substance which causes a false - positive reaction , by a method including the steps of : inactivating the limulus reaction - activating substance , if any , in the biological product , by exposing the biological product to a surfactant at a temperature ranging from the surfactant &# 39 ; s freezing point to 50 ° c . ; and measuring the amount of endotoxin present in the biological product , using a limulus reagent .