The rate of multiplication of PVM in the mouse lung is relatively constant, averaging 7.9-fold per day with but slight variations, irrespective of the amount of virus inoculated. Similarly, the rate of increase in the amount of pneumonia is relatively constant, averaging 4.7-fold per day, even though the quantity of virus inoculated is varied over a wide range. It follows that viral multiplication proceeds 1.7 times more rapidly than does extension of the pathological lesion, both reaching limiting maximal values in periods which are predictable from the amount of virus inoculated. From the concentration of virus determined at any time during the incremental period, the amount of pneumonia present earlier or later in the incremental period can be computed with considerable precision. The results support the postulate that the extent of the pathological lesion is dependent upon the degree of viral multiplication.

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