Concomitant immunity is one of the most important, yet one of the most baffling, phenomena confronting tumor immunologists. We hypothesize that concomitant immunity is local immunosuppression in the face of systemic immunity and is produced by the local interaction of the host's immune apparatus and the tumor. We plan to determine if immunosuppressive factors are extractable from tumors or tumors and surrounding tissue. We also plan to see if immunosuppressive factors are produced by tumors, or combinations of tumors and lymphoid cells grown in tissue culture. Immunosuppression will be assayed by inhibition of: (a) mitogen or antigen induced lymphocyte proliferation, (b) antibody synthesis in tissue culture, (c) in vitro antitumor (target cell) lymphocytotoxicity, (d) antibody production in vivo, (e) the immune response in vivo to a different tumor.