The majority of rabbits receiving intradermal, intraperitoneal or intravenous injections of moccasin venom became refractory to the development of the Shwartzman phenomenon. An incubation period of about 14 days was required for their resistance to develop. The incidence of refractory animals was inversely proportional within limits to the amount of toxin given intravenously to elicit the Shwartzman phenomenon. The intravenous route was the most efficacious in developing refractivity. The refractory state was still present 44 days after the primary injection of moccasin venom.

Rattlesnake venom was not efficacious in inducing a refractory state.

The refractory animals did not show a changed reaction to moccasin venom in the concentrations used.

No circulating antibodies could be demonstrated to explain the refractory state.

Antivenin had no effect on the course of the Shwartzman phenomenon.

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