Interference between a virulent and an avirulent strain of Sendai virus was studied by plaque morphology and analysis of viral protein and RNA. On simultaneous infection a virulent, large plaque-forming strain (RL) was inhibited by an avirulent, small plaque-forming strain (RS). The interference was dose-dependent and decreased with u.v. irradiation of RS. One infectious particle was sufficient to induce the interference. SDS-polyacrylamide gel electrophoresis showed that interference was detectable in the synthesis of viral P protein; this was abolished when RS was u.v.-irradiated. Both growth and P protein synthesis of RL was restricted by superinfection with RS when this was done within 4 h after infection of RL, but the interference decreased gradually after this period and was not detectable after 8 h. Cycloheximide prolonged the period susceptible to superinfection by RS.