The overall objective of the proposed research is to understand the pathogenesis and immunologic features of Marek's disease (MD) in chickens. Specific objectives include: 1) identification and characterization of lymphocyte subpopulations which are the targets for latent, cytolytic or transforming infection with the MD herpesvirus. 2) Characterization of the MD tumor cells in terms of biologic and antigenic features, and 3) systematic identification of effector systems and potential targets in immune responses important in resistance and vaccinal immunity.