A purification scheme is described for the M-protein of Sendai virus and an electron microscope study of the isolated protein is presented. The protein exists as subunits of 6 nm in diam., which possess a central hole; the subunits may be dimers of the polypeptide. They are able to form filamentous aggregates which wind around one another to form a helical structure. It is suggested that these filaments may be the form adopted by the protein in the virus, the filaments lying parallel to one another just beneath the virus membrane to form a shell, but that the helical form is likely to be a property only of the isolated protein.