Influenza A virus, PR8 strain, increases in amount in the infected mouse lung at a relatively constant rate. When more than 25 M.S.50 doses of virus is inoculated, the rate of multiplication appears to be independent of the amount of virus introduced; has a value of 1,100-fold increase per day. The rate of increase in the pulmonary lesions induced by infection of the mouse lung with PR8 also appears to be relatively constant and independent of the amount of virus inoculated; has a value of 8.5-fold increase per day.
The essential variables in the PR8-mouse lung system appear to be equated satisfactorily by functions which were derived previously (4) during a similar quantitative investigation on pneumonia virus of mice (PVM). Evidence in support of the hypothesis that the processes of multiplication of PR8 and PVM are different in the mouse lung is presented.