Turkeys inoculated with spleen extracts from lymphoproliferative disease (LPD)-affected birds developed viremia, followed by typical LPD lesions. Electron microscopy and biochemical characterization established that the virus present in the blood of infected turkeys is a type C retrovirus. The viral particles possess a buoyant density of 1.17 g/ml in sucrose gradients; they contain high-molecular-weight RNA and an RNA-instructed DNA polymerase with efficient exogenous and endogenous activity. The LPD virus polymerase is preferentially activated by magnesium ions. Cross nucleic acid hybridization assays revealed no sequence homology between the viral genome of LPD and avian myeloblastosis virus or reticuloendotheliosis virus, thus indicating that the LPD virus belongs to a distinct group unrelated to the avian leukosis-sarcoma virus complex or to the reticuloendotheliosis virus group.