The pattern of integration of spleen necrosis virus (SNV) DNA in DNA from a large population of SNV-infected chicken cells was studied by nucleic acid hybridization with iodinated viral RNA by the blotting technique of Southern. SNV DNA was found to be integrated at multiple sites in acutely infected chicken cells. Concomitant with the transition from acute to chronic infection, a shift in the pattern of integration was observed. The majority of integrated SNV DNA found in acutely infected cells was absent from chronically infected cells. This result is consistent with the hypothesis that the cell death that occurs after infection of avian cells with reticuloendotheliosis viruses is a consequence of the multiple integrations of the provirus. Viral DNA was also integrated at multiple sites in chronically infected cells. However, infectious viral DNA molecules in chronically infected cells migrated in a uniform manner in agarose gel electrophoresis after EcoRI digestion (which does not cut viral DNA), indicating that not all integrated SNV copies are equally infectious.