The Influenza virus possesses two modules: internal ribonucleoprotein (RNP) containing the viral genome RNA and external lipid envelope with transmembrane ionic channel protein M2 and embedded glycoproteins hemagglutinin (HA) and neuraminidase (NA) forming surface spike ends. These modules are combined in a whole virion by the matrix protein M1. The effect of the acidic pH 4,2-4,5 on the influenza virus grown in MDCK-H cells was tested. The A/Aichi/68 (H3N2) virus synthesized in MDCK-H cells was shown to contain uncleaved HA0 (m.w. 78 kD) and provide low infectivity. This virus was resistant to acidic medium and non-permeable to the phosphotungsten acid (PTA) used in electron microscopy as a contrast stain, and did not reduce infectious potential after acidic treatment. The trypsin-activated virus containing cleaved HA1 (56 kD)+HA2 (22 kD) was sensitive to acidic exposition resulting in the appearance of permeability to PTA, reduction of infectivity, enhancement of the M1-RNP interlink. These data indicate that the structural form of the cleaved HA1 +HA2 surface hemagglutinin coordinates a transmembrane interaction between surface and internal virus components.