Investigations into the role of surface antigen of B and T lymphocytes in determining inherited differences in the susceptibility of chickens to viral oncogenesis will be continued. In a new experimental design, F3 chickens of all nine genotypes at the Bu-1 and Th-1 loci will be compared for susceptibility to oncogenesis by lymphoid leukosis and sarcoma viruses, and for tumor regression. Similar tests will be made for Marek's disease herpesvirus. The same design will be used to test and confirm associations of Bu-1 and Th-1 genotypes with cellular and humoral immune responsiveness, and with lymphoproliferative potential, in extension of our preliminary findings. The mechanisms of all the above types of association will be investigated by in vivo cell transfers, and by in vitro studies of lymphocytes including antigen co-capping, cell interactions, and proliferative responses, as affected by specific Bu-1 and Th-1 antisera. Further studies will be made to test our preliminary finding of a major role for bursal target cell genotype in resistance to lymphoid leukosis, and to delineate the contribution of immune surveillance. In these, bursa-inhibited chicks of one susceptibility genotype will be repopulated with bursal cells from chickens of a contrasting susceptibility genotype and studies for pathological changes following virus inoculation.