Slab polyacrylamide gel electrophoresis of Influenza-virus A strains varying in antigenic composition of the envelope and in the natural host species showed the nucleprotein and membrane proteins to have a standard molecular weight of 58,000 and 24,000 daltons, respectively. The electrophoretic mobility and molecular weights of the light and heavy chains of haemagglutinin varied in the strains under study. The differences observed persisted on passaging and were transferred upon genome recombination.