The involvement of the immune system in the host response to tumors is complex and involves both lymphocytes and lymphocyte-derived mediators, as well as inflammatory cells and various other agents such as complement. These activies are not confined to cytodestructive processes; recent studies demonstrate that the migration characteristics of tumor cells may be modified by immunologically derived substances. The multiplicity of possible immune system-neoplastic cell interaction is unfortunately balanced by the multiplicity of mechanisms that serve to interfere with an effective immune response to tumors. These mechanisms may reflect pathologic derangements of normal immunoregulatory processes or may involve the production of suppressive substances by the tumors themselves. In either case, a number of genetic and other predisposing factors must contribute to the ultimate resolution of the battle between host and tumor. Successful strategies for immunologic intervention must take these parameters into account.