The nonglycosylated membrane protein (M) of Sendai virus was purified from virions and conditions were found under which the protein assembled in vitro into three types of ordered structures: narrow tubes, wide tubes, and sheets. These structures were examined by high resolution electron microscopy by using negative staining and metal shadowing techniques. The tubes and sheets are formed from strands 7.2 nm wide that are composed of annular subunits. The wide tubes appear to be formed by the rolling of a sheet into a cylinder in which the 7.2-nm strands are inclined with a pitch of 26-33 degrees and have a left-handed orientation. In addition to the strong reflections corresponding to the 7.2-nm spacings generated by the strands, optical diffraction patterns also showed weak reflections that could be indexed on a lattice corresponding to real-space lattice constants of 7.6 nm and 5.3 nm, with an included angle of 71 degrees. The dimensions and arrangements of these structures formed in vitro are strikingly similar to those of ordered arrays of particles found by others to be associated with the inner surface of the plasma membrane of infected cells. The results support the concept that ordered arrays of M protein, similar to those assembled in vitro, are involved in the assembly of the virus particle by budding from the cell membrane and that they provide specific recognition sites for the viral nucleocapsid at the cytoplasmic surface of the plasma membrane.