Strains of Poliomyelitis Virus

Since the early days of the discovery of the virus of poliomyelitis it has been known tha t quant i ta t ive differences in virulence for the monkey exist between recently isolated strains, and strains which have passed through successive monkeys. Only within the last few years has it been clearly demonstra ted by the work of Burne t and Macnamara (1) in Australia, tha t immunologic differences exist between such strains, which might be termed quali tat ive differences, in tha t they do not seem to be confined to those of virulence alone.