The control of neoplastic disease requires a knowledge of the interaction of the carcinogenic agent, such as virus, and the cell and those factors which influence the development and control of the disease in the host. We are studying a model system for this process, developed in our laboratory, which consists of a virus induced leukemia in mice that spontaneously and reproducibly regresses. Studies on the virus and host factors have included the immunological, virological, biochemical and ultrastructural events which occur during leukemogenesis and regression, and are continuing. Current emphasis is on the specific virus components which are associated with, and responsible for regression; a detailed comparison of virions which induce a leukemia which regresses and those which do not is in progress. The immunological reactivity which we have shown to be necessary for regression to occur is also being analyzed and will be correlated with our hematological and pathological findings in leukemic and regressed animals.