Analysis of biological heterogeneity in different mice strains in dose dependent manner. Pandemic H1N1 viruses (NC2 and NC8) were compared in C57/BL6 and BALB/C mice in dose dependent manner. Weight losses (a, c) and lethality (b, d) are shown for animals (n = 10/group) infected with varying concentrations of viral strains (103, 104, 105 and 106). Log rank sum test shows significant differences in mortality curves between NC2 and NC8 irrespective of inocula and mice strain (P < 0.0001). ). (a, c): Inter-group comparisons in weight loss shows significant difference NC2 vs NC8 groups (P < 0.0001), C57/BL6 vs BALB/C; NC2_ 104 (P < 0.0001), NC2_105 (P < 0.0001). (b, d): Survival differences are also significant in C57/BL6 vs BALB/C after NC2 infection: 104 (P < 0.0028), 105 (P 0.0005), 106 (P 0.0669).