The objective of this proposal is to employ the feline anemia model to investigate the mechanisms of acquired bone marrow aplasias and to develop and evaluate therapeutic regimes for aplastic anemias. In order to compare the pathogenesis of viral vs. chemically induced chronic marrow suppression, a model of benzene-induced nonregeneraative anemia in cats also has been developed. The specific aims of this study are: (1) to culture and characterize the erythropoietic (CFU-E) and myelopoietic (CFU-C) marrow progenitor cells in normal cats and cats with aplastic anemia induced by FeLV or benzene, (2) to correlate marrow erythroid and myeloid cell growth and differentiation with the presence of FeLV antigens and FOCMA, (3) to detemine whether humoral factors, lymphoid cells, or leukemia virus-associated proteins mediate marrow suppression in vitro and in vivo, (4) to characterize hemo-suppressive factors and/or cells and determine their relationship to the immunosuppression in preleukemic and anemic cats, and (5) to develop methods to treat aplastic anemia by eliminating suppressors and infusion autologous cryopreserved pre-disease marrow cells or marrow cells "cured" of aplasia in vitro.