Monkeys (Macacus rhesus) which have survived a mild attack of experimental poliomyelitis following the injection of a weak strain of virus 1, develop second attacks when given a minimal paralyzing dose (M.P.D.) of a more virulent heterologous strain; 2, are protected when given a second M.P.D. of a weak heterologous strain.

Monkeys which have survived a severe attack following the injection of a virulent strain of virus always survive when given a M.P.D. 1, of a weak heterologous strain, or 2, of a virulent homologous or heterologous strain. These animals, however, may develop the disease again, if given massive doses of a homologous or heterologous strain of highly virulent virus.

1

This study was supported, in part, by a grant from the Marion R. Spellman Fund, the Cleveland Foundation.

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