WHEN a watery suspension of Gram-negative organisms is heated at 80° C for 1 h, the endotoxin is liberated into the suspending fluid, giving it a dichroic appearance; a method of preparing endotoxin on a large scale by recovering it from such a supernatant was described earlier1. In that method concentration was effected by evaporation under reduced pressure. This method, improved and modified to avoid this, is described here.