The search for spontaneously occurring or experimentally developed avirulent strains of poliomyelitis virus suitable for human immunization calls for fundamental knowledge of the conditions which favor the appearance and segregation of variants or mutants, as well as of the properties by which they may be characterized. Previous demonstrations of modifications of the paralytogenic activity of poliomyelitis virus in monkeys have depended on continued propagation in tissues of another host. Theiler (1) was the first to report that, after 50 rapid intracerebral passages in mice, the Lansing strain no longer produced signs of poliomyelitis in 8 intracerebrally inoculated rhesus monkeys.In other laboratories (2, 3) the same strain of virus still produced paralytic poliomyelitis in rhesus monkeys after approximately 200 or more intracerebral passages in mice. Other strains of poliomyelitis virus have also been found to lose much, although not all, of their paralytogenic activity in monkeys after passage in mice, cotton rats, or hamsters, while gaining in virulence for the new hosts (4-7). When it was shown that cultivation of the Type 1, Brunhilde strain in human non-nervous tissue resulted in a marked reduction of its paralytogenic activity in monkeys (8), the question arose whether the effect was due to propagation in the tissues of another host or to propagation in non-nervous tissue. Accordingly in the design of the present studies it was decided to keep the host constant and to vary the conditions of cultivation.Viruses possessing a high original virulence for the cynomolgus monkey by various routes were selected for study. The purpose of the present communication is to present data which indicate that serial propagation in cynomolgus

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