A COMPARATIVE STUDY OF RECENTLY ISOLATED HUMAN STRAINS AND A PASSAGE STRAIN OF POLIOMYELITIS VIRUS.

1. Confirmation of the qualitative differences which exist between so called human and passage strains of poliomyelitis virus has been established by the following observations. (a) The experimental disease induced by two human strains usually failed to protect monkeys against a subsequent infection by a passage strain, and in the few instances in which the reverse experiment could be tried a similar lack of protection was observed. (b) In some human sera the neutralizing power for a human strain differed qualitatively from the neutralizing power for a passage strain. 2. The time interval between the intracerebral inoculation of heterologous strains has been found to be an important factor bearing upon the results of the reinoculation experiments reported. Within the intervals used, the greater the period between the original infection and the reinoculation with a heterologous strain, the less was the degree of cross-immunity observed.