Intraperitoneal or intravenous inoculation of live or inactivated influenza virus induced two characteristic responses of the peripheral leukocytes in mice, an early appearing leukopenic response and late appearing lymphopenia. The former response usually developed and subsided within several hours, though the change in leukocyte population was fairly complicated depending upon the activity of the inoculated material, while the latter began several hours after inoculation and reached its minimum level in 10 to 20 hr. The agent responsible for the former may be virus pyrogen, while the latter seems to be caused by some substance (s) other than that.
The early appearing leukopenic response was similar to that due to bacterial endotoxin in respect to the characteristic pattern of the change in peripheral leukocyte population, though it was relatively easy to distinguish one from the other by the length of the latent period and by the heat stability of the causative agent. Live or inactivated influenza virus causing the early appearing leukopenic response was found also to have the mouse body weight-decreasing toxicity.
The significance of these findings in the laboratory control test of influenza vaccine for untoward reactions often observed in human inoculated with some inactivated influenza vaccines was discussed. The possible roles of the two agents, virus pyrogen and endotoxin, in the febrile response were mentioned.