The number of leukocytes in BALF from SD/09-infected mice showed an increase from day 4 p.i. ( Table 2 ). The BALF of virusinfected mice on day 6 p.i contained 1.1610 6 cells/ml and was significantly different from the 1.6610 5 cells/ml observed for PBSinoculated mice (p,0.001). These data indicate a dramatic increase in inflammatory cells in the lungs of SD/09-infected mice. To quantify the immune cell subpopulations responding to viral infection, we next determined cell differential counts in the infected lungs by Wright staining. Compared with PBS-inoculated animals, mice infected with SD/09 virus exhibited an increase of neutrophils from 4 days p.i., and the peak was 18-fold greater than that of the control group on day 8 p.i. Leukopenia was detected on day 4 p.i., and was statistically significant on day 6 p.i. (p,0.05); the lowest value appeared on day 8 p.i. (p,0.001). Furthermore, differential blood counts revealed that the number of lymphocytes sharply decreased in infected mice. The lowest number of lymphocytes observed occurred on day 8 p.i. (Figure 4B ), which dropped to ,20% of the control group number (p,0.001).