1. Adult rhesus monkeys possess frequently a greater resistance to experimental infection with poliomyelitis virus than young monkeys as indicated by the prolongation of the incubation period and the content of feeble neutralizing substances in the normal serum.

2. Virucidal substances can be demonstrated in the normal serum of subadult chimpanzees.

3. Immature monkeys prepared with glandular anterior pituitary hormones and anterior pituitary-like principles from the urine of pregnant women are protected only in exceptional instances against intracerebral infection with poliomyelitis virus. However, the serum of such prepared animals frequently acquires the property of inactivating the virus in vitro.

4. Active immunization with live poliomyelitis virus renders the immunized monkeys resistant to intracerebral infection in a higher percentage of the cases than does preparation with hormone.

5. The relative merits of non-specific enhancement of resistance and of specifically acquired immunity are discussed in their relation to the probable mechanism of resistance to poliomyelitis in man.

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