The matrix protein M and the nucleocapsid protein N were isolated from vesicular stomatitis virus and reconstituted into artificial phospholipid vesicles. While the M protein could be reconstituted into phospholipid vesicles, the N protein had no affinity for lipid vesicles. The N protein could, however, associate with phospholipid vesicles in the presence of M protein. Identical results were also obtained when an in vitro system synthesizing M and N proteins was used for reconstitution. The results suggest that M protein is involved in virus maturation by interacting with the viral envelope and the N protein of the nucleoprotein core.