Electron micrographs of stain-penetrated influenza virus particles show a variety of internal structures that all consist of units of 4 X 4 nm. Isolated influenza virus M-protein, that is reassociated with lipid, forms very similar structures on liposomes. We therefore conclude that the 4 X 4 nm unit is M-protein. We further argue that the M-protein is not arranged with icosahedral symmetry.