A study of the multiplication of PVM in the mouse lung yielded evidence in support of the hypothesis that discrete cycles occur. The cycle is characterized by a latent period of about 15 hours, an incremental period of 10 to 15 hours and a total duration of 24 to 30 hours. The yield of virus per cycle is approximately 16-fold.

A large proportion, approximately 90 per cent, of PVM is lost on intranasal inoculation and cannot be recovered from the lung. Of the virus which reaches the lung, almost all becomes non-infective within 12 hours even though a considerable amount retains the capacity to cause hemagglutination. During the incremental period infective virus appears as soon as any increase in hemagglutination is demonstrable.

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