Injection of a dual tropic virus (DTV) isolated from a T cell lymphoma AKR/J mice into the thymus of 14 day old AKR/J puppies accelerates lymphoma development; 90-100% of the injected mice develop the disease within 120 days. In contrast a cell free centrifuge CFC-666 prepared B cell lymphoma of AKR/J origin injected into the thymus of 14 day old AKR/J mice failed to accelerate T cell lymphomagenesis and actually prevented the spontaneous T cell lymphoma development. However, 50% of the treated mice developed B cell lymphoma with a latency of 417 + 18 days. DTV injection induces amplification of thymic expression of MuLV related antigens, besides changes in thymus subpopulation. Such changes emerge spontaneously in 5-6 month preleukemic AKR/J mice (at the time of spontaneous DTV formation in the thymus). These changes in the thymus were not observed following CFC-666 injection. We assume therefore that CFC-666 interferes with spontaneous DTV formation that contributes to T cell lymphomagenesis.