Aged BALB/c mice (greater than 18 months) have previously been shown to begin to express endogenous retrovirus and to develop malignant lymphomas and leukemias. The purpose of these pilot studies are to expand upon these observations by characterizing the cell types involved and the role of a leukemogenic endogenous retrovirus in the development of malignant lymphoid disease. This system may serve as a model for the development of leukemias in aged humans since at least one retrovirus, HTLV-1, has been found to be associated with the development of adult T cell leukemia.