1. The effect of bacterial pyrogens on plasma zinc concentration in mice was studied as a method of bioassay for these substances.

2. A dose-related depression of plasma zinc concentrations was observed 4 h after intravenous injection of doses of 0.05-500 ng of purified endotoxins of Salmonella abortus equi, Shigella dysenteriae and Escherichia coli. Responses were dose-dependent and reproducible for each endotoxin in five strains of mice.

3. Tolerance to endotoxin could be induced in mice by injection of doses of 500 ng but was not seen with doses of <5 ng, even after repeated treatment.

4. Dinitrophenol, a metabolic inhibitor, and amphetamine, a metabolic enhancer, did not affect plasma zinc levels, indicating that changes in basal metabolic rate do not affect the outcome of this assay. The effects of pyrogenic materials other than endotoxin on plasma zinc levels were also tested and the results suggest that measurement of hypozincaemia in mice provides the basis for a simple, practical and inexpensive test for endotoxins and other pyrogens.

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