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Ultra-centrifugation of emulsions from suckling mouse brains infected with the MEF1 strain of poliomyelitis virus separates a non-infective antigen, or soluble antigen, from infective virus. This antigen is responsible for most of the complement fixation and explains the high titres obtained. On the other hand, the same virus, and also Mahoney, Type 1 poliomyelitis virus, grown in monkey kidney tissue culture, fail to produce this soluble antigen, and this is probably a factor in the low complement-fixing titres obtained in tests with these fluids. The author would like to express her appreciation of Miss M. Butchart's valuable technical assistance.
Golda Selzer (1958) studied this question.