Table 1. Properties of recombinant influenza viruses used in this study.

Virus*	Titer†, plaque-forming units/ml	% Weight loss‡	Lung titers§	LD50¶
Parental WSN virus	2.2 × 107	28.4	6.7 ± 0.2	2.5
1918 HA/NA	2.1 × 107	20.9	7.3 ± 0.1	2.75
1918 HA/NA/M	1.4 × 108	28.8	7.9 ± 0.2	2.75
1918 HA/NA/M/NS	2.1 × 107	23.7	7.3 ± 0.2	3.25
1918 HA/NA/M/NS/NP	1.4 × 108	24.5	7.4 ± 0.2	1.75

^*All viral genomic segments were derived from the WSN virus unless otherwise indicated.

^†Titer of virus stocks prepared on MDCK cells.

^‡Average percent weight loss on day 4 p.i. (five mice per group).

^§Average lung titers of four mice on day 4 p.i. expressed as elD₅₀/ml ± SE.

^¶Expressed as the log₁₀ plaque-forming units required to give 1 LD₅₀.