1. Attempts to separate by processes of physical segregation, i.e. ultrafiltration, ultracentrifugation, or dialysis, from live SK murine poliomyelitis virus a non-pathogenic agent capable of interfering with simian poliomyelitis virus were unsuccessful. Neither was it possible to convert live SK murine virus into a non-pathogenic interfering agent by processes of chemical inactivation, i.e., phenolization or formalinization.

2. Preparations of SK murine virus, which had been markedly attenuated by ultraviolet irradiation, gave evidence of having retained some interfering power in rhesus monkeys.

3. MM murine poliomyelitis virus interfered, both in mixture tests and by peripheral administration, with two simian strains of poliomyelitis virus. With adequate amounts, distinct protective effects could be obtained in rhesus monkeys which had received murine virus (animal passage or tissue culture virus) up to 48 hours after intracerebral infection with simian poliomyelitis virus.

4. Theiler's virus of spontaneous mouse encephalomyelitis, when tested in mixture with simian poliomyelitis virus, gave some evidence of irregular and low grade interference. Interference could not be shown conclusively in experiments to prevent poliomyelitic infection or to modify its effects.

5. The nature of the interfering agent present in murine virus is discussed.

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