Lipopolysaccharides from gram-negative bacteria, i.e., endo toxins, when administered parenternally, induce profound cardiovascular collapse and ultimately lethality in shock in various animals ( 1 , 2). However, in comparison to other species, rats are highly resistant to the physiopathological effects of endotoxemia, e.g., Zweifach (3) has compiled the following relative LD75 for circulatory collapse: cat, 1; rabbit, 2.5; dog, 8; guinea pig, 20; and the rat, 300. Thus, despite the great utility of rats in various experimental shock models, the high resistance to endotoxin has detracted from use of rats in analysis of the cardiovascular events in endotoxin shock.Recently, Selye, Tuchweber, and Bert6k (4) reported that a single intravenous injection of lead acetate markedly sensitized rats to lethal endotoxin shock and suggested use of this synergism as a convenient bioassay for minute amounts of endotoxin. The present study evaluated the use of the lead-treated rat in the bioassay of the ability of rat organ homogenates to inactivate endotoxin.Methods. Male Sprague-Dawley rats of the Holtzman strain, with a body weight range of 300 -t-20 g, were maintained on Purina Chow and water ad libitum at a constant temperature of 24 t 1" for 7-10 days prior to use. Endotoxin was purchased from Difco Laboratories, Detroit, Michigan, as the Boivin lipplysaccharide preparation from Salmonella enteritidis. Endotoxin was prepared daily in 0.0% NaCl. Lead acetate (Mallinckrodt Chemical Works, St. Louis, Mo., No. 5684) was prepared daily in distilled water.Organs for homogenates were obtained from rats sacrificed by a stunning blow to the