We attempted to quantify the protective potential of polyclonal IgG antibodies specific to the ectodomain of M2 protein (eM2) of influenza A virus (IAV) against lethal influenza infection of mice. For this purpose, eM2 conjugated with keyhole limpet hemocyanin (KLH) or KLH alone were administered with Freund's adjuvant intraperitoneally (i.p.) to BALB/c mice. IgG antibodies specific to the KLH-eM2 conjugate (anti-KLH-eM2 IgGs) and KLH (anti-KLH IgGs), respectively, were purified from ascitic fluids. Analysis of the preparation of anti-KLH-eM2 IgGs by ELISA revealed that it contained about 25% of anti-eM2 IgGs and 75% of anti-KLH IgGs. Taking into account this finding mice were passively immunized by intravenous route with 320, 160, 80, and 40 µg of anti-eM2 IgGs per mouse, respectively, while 320 µg of anti-KLH IgGs were used in control. Following subsequent infection with 3 LD50 IAV the survival of mice was determined. An absolute protection (100% survival) was obtained with 320 µg of anti-eM2 IgGs, and a relatively strong significant protection (~80% survival, p = 0.024) with 160 µg. The amount 160 µg of IgGs represents approx. 100 µg IgGs per 1 ml of blood.