The ability of defective-interfering (DI) particles of vesicular stomatitis virus (VSV) to induce interferon was studied in relation to the amount of rapidly reannealling (snapback) [±] double-stranded sequences in their RNA.Five DI particles propagated in BHK-21 cells were analyzed: 2 DI particles generated by undiluted passages of cloned wild-type VSV (Indiana); 2 DI particles generated by serial undiluted passages of culture fluid from L cells persistently infected with VSV; and D0-011, a DI particle with [±] snapback RNA, which is known to be a potent inducer of interferon.Induction of interferon in L cells by these DI particles was not proportional to the amount of [±] sequences in their RNA.DI-011 (26-37% [±] RNA sequences) induced a signficant amount of interferon at a multiplicity of infection of 1 DI particle per cell.In contrast, the 2 DI particles from wild-type VSV (43-54% [±] RNA sequences) were 20- to 30-fold less efficient inducers of interferon than DI-011.Furthermore, the 2 DI particles (1-4% [±] RNA sequences) generated from L cell carrier cultures were only slightly less efficient inducers of interferon than the wild-type DI particles.Apparently, a population of DI particles which contains [±] RNA is not selected in L cells persistently infected with VSV.