At intermediate concentrations of DMSO the yields of infectious virus and biologically active hemagglutinin and neuraminidase of an influenza A virus (fowl plague virus) and of reassortants therefrom are enhanced severalfold, even though viral protein synthesis is not significantly affected. A corresponding enhancing effect was also found with New castle disease and Semliki Forest viruses. At elevated concentrations of DMSO virus yield decreases, and under these conditions the synthesis of the late influenza virus proteins is specifically inhibited. The results indicate that DMSO can facilitate the assembly of virus particles, and viral components, which are normally produced in surplus amounts, now contribute to the maturation of infectious particles.