Rabbits given an intravenous injection of an 18 to 24 hour broth culture of E. coli commonly died within 28 hours. The injection of a 4 hour broth culture of the same strain of E. coli containing equal numbers of living bacilli produced only an occasional death.

Initial clearance rates and subsequent bacteriemias were similar in animals receiving either culture. Study of changes in circulating leukocytes or the temperature response to washed bacterial cells or culture filtrates failed to reveal obvious differences in host response to young or old cultures.

It was found that both living bacterial cells and some substance or substances present in culture filtrates were required to produce subsequent death. The injection of whole old cultures containing both these factors produced hypothermia instead of the endotoxin type fever response which followed the injection of whole young cultures.

Subsequent experiments revealed that this hypothermia appeared to be secondary to a period of transient but profound shock which occurred soon after the injection in rabbits receiving old cultures. No significant alterations in arterial pressure accompanied injections of young cultures.

Evidence is presented which suggests that this period of hypotension rendered animals more likely to die with a persisting bacteriemia tolerated without event by non-shocked animals. The mechanisms which operate to increase mortality during the post shock period are as yet unclarified.

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