1. The murine strain of SK poliomyelitis virus interferes with the propagation in rhesus monkeys of SK, Aycock, and RMV poliomyelitis monkey virus.

2. This interference is demonstrable by intracerebral injection of mixtures of murine and monkey virus prepared in vitro as well as by separate injection of the two viruses by diverse routes.

3. Mixture tests carried out with graded doses of murine and monkey virus show that 0.5 cc. of a 10 per cent suspension prepared from the brains of paralyzed mice is capable of counteracting at least 100 minimal paralyzing doses of two strains of monkey virus.

4. No interference was demonstrable with suspensions of brains infected with murine virus which had been inactivated by heating for ½ hour at 75°C., or with suspensions prepared from normal mice, or with brain suspensions prepared from mice infected with herpes virus.

5. When murine virus is introduced into monkeys by the intravenous route, before or after intracerebral infection with monkey virus, distinct prophylactic or therapeutic results may be obtained.

6. Analysis of the figures shows that the success of interference depends upon (a) the size of the infecting dose of monkey virus, (b) the amount of murine virus injected, and (c) the choice of proper intervals between the injection of monkey and murine virus.

7. The mechanism of the interference phenomenon here described is discussed in the light of the available data.

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