The capacity of an egg-grown Sendai virus preparation to induce interferon in the human lymphoblastoid cell Namalwa is dependent on its passage history. Virus which has been serially passaged at high dilution is a poor inducer, whereas virus serially passaged undiluted is a good inducer. Such a good induce preparation has a low infectivity to haemagglutination ratio as the result of a high content of defective-interfering (DI) particles. Using DI particles purified on glycerol gradients, it is shown that for the induction of maximum interferon titres both infectious and DI particles are required. DI particles alone induce little or no interferon. Addition of DI particles to fully infectious Sendai virus preparations increased the interferon yield obtained from Namalwa cells some 60- to 100-fold.