Additional evidence is presented affirming the role of the intestinal pool of endotoxin in producing irreversibility in prolonged hemorrhagic shock. The fact that coliform-free rabbits tolerate exposure to a degree and duration of hemorrhagic shock which is lethal for rabbits that possess the normal flora, and that these tolerant rabbits lose their tolerance when E. coli are introduced into the gut several hours before inducing shock, demonstrate the critical importance of the size of the pool of endotoxin. That there is a proportionality between the size of the pool of endotoxin and the tolerance of hemorrhagic shock is suggested by the survival rate of several series of coliform-free rabbits fed E. coli by gavage. The rate was less the more firmly the E. coli were reestablished in the flora. The presence of the usual number of coliform bacteria in the intestinal flora does not mean the presence of the usual amount of endotoxin in these bacteria. The amount of endotoxin depends not only on the size of the population, but also, as our own experience demonstrates, on the particular ecological factors extant at any particular time which govern the amount of endotoxin elaborated by any given strain or strains of coliform bacteria.