1. SK murine virus maintained over more than 200 serial mouse passages increased in virulence for mice from an initial intracerebral titer of about 1:1 million to a maximum titer of not less than 1:1 billion dilution activity.
2. Following intracerebral injection with murine virus of remote mouse passages, 5 of 13 rhesus monkeys developed a characteristic encephalitic syndrome. Repeated intravenous injection of massive doses of virus caused localized flaccid paralysis in 2 of 14 monkeys.
3. Intracerebral injection of graded doses of murine virus into mice of different age groups caused fatal paralysis in young and old animals alike. Infection with small doses of virus by peripheral routes, while uniformly fatal to young mice, was followed by survival of almost half of the old mice.
4. The incubation period of the disease in young mice infected intracerebrally with a standard dose of murine virus, when studied throughout the period of 1 year, was found considerably lengthened during the summer months.
5. Cross neutralization tests furnished no evidence for any serological relationship between SK murine virus and lymphocytic choriomeningitis virus. Theiler's virus was found to be neutralizable by antimurine horse serum and, to a lesser extent, by concentrated antipoliomyelitis horse serum; however, such inactivation, in both cases, was distinctly inferior to that occurring with SK murine virus. On the other hand, no neutralization whatsoever was obtained between SK murine virus and normal adult mouse serum, whereas the same serum completely neutralized Theiler's virus. Mice surviving infection with Theiler's virus, though acquiring immunity to this virus, remained fully susceptible to reinfection with SK murine virus.
6. Neutralization tests with SK murine virus against poliomyelitis-convalescent monkey sera gave irregular results, but neutralization of murine virus occurred regularly with a hyperimmune antipoliomyelitis horse serum. Hyperimmune antimurine horse and rabbit sera, on the other hand, failed to inactivate three strains of monkey poliomyelitis virus (SK, RMV, Aycock) by intracerebral tests in monkeys. The same sera inactivated murine virus in mice by intraperitoneal, but not by intracerebral injection of virus-serum mixtures.
7. The identity of SK murine virus and its relation to other rodent strains of poliomyelitis virus is discussed on the basis of the available data.