A comparison of the growth curves of adapted and unadapted lines of the same strain of Type A or Type B influenza virus in mice has demonstrated that the unadapted lines of virus exhibit a lag phase of growth lasting approximately 12 hours during which little or no multiplication of virus can be demonstrated. Multiplication by the adapted line begins earlier and may be evident as soon as 6 hours after inoculation. Following the lag phase, the rates of multiplication of both lines of virus were indistinguishable when dilute inocula were used. The maximal infectious titer of the unadapted line was lower than that of the adapted line of the same strain and this disparity was increased following inoculation with large doses of virus. The shape and slope of the growth curves of influenza virus in mice were shown to be determined by the line, strain, and concentration of virus used for inoculation. The significance of these findings has been discussed.

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