This is a proposal to study the mechanisms of recovery from infection and maintenance of immunity and its impairment by lymphomas and leukemias and by immunosuppressive agents used for cancer therapy. Lymphomas, radiation and chemotherapy regimens permit the occurrence of relapse of chronic infection, or reinfection. The main emphasis is to be on the little understood mechanisms of cell-mediated immunity to intracellular infections and its suppression. In the analysis of cellular immunity, irradiated and reconstituted animals will be used, and also thymectomy and treatment with corticosteroids, cyclophosphamides, and other metabolic inhibitors. Extensive use will be made of cell cultures, employing lymphocytes and macrophages from normal and immune donors, comparing also homospecific and heterospecific immunity in activated cell populations. Besnoitia jellisoni and Toxoplasma gondii will be studied. In the intact hosts, immunity and hypersensitivity findings will be distinguished. Observation in cell cultures will be compared with findings in the intact hamsters. Apparent acquisition of immunity by nonlymphoid cells will be examined. Results from hamsters, and from infection models studied by other investigators, will be compared for their model value in explaining clinical problems of recovery from infection and the maintenance of immunity in humans, especially the immunosuppression experienced during cancer chemotherapy with corticosteroids, cyclophosphamide and irradiation.