An ecotropic type C retrovirus designated as D-1 MuLv and a xenotropic virus were detected in lymphoma of SJL/J mice. The ecotropic virus titer was high in mice with adenocarcinomas. The xenotropic virus, on the other hand, can be detected at 6 months of age, not earlier. Cross neutralization and competitive radioimmuno assays indicate that ecotropic virus is closely similar to AKR-type ecotropic virus and the xenotropic virus to Balb-2. Isolation of reverse transcriptase-p30 complex is in process.