Viruses can render services to mankind. 1. Retroviruses pinpoint and transduce cellular oncogenes. 2. Retroviral vectors can introduce antioncogenes (the RB gene) into malignant cells thus rendering the recipient cells nonmalignant. 3. Oncolytic viruses lyse tumor cells. 4. Parvoviruses replicate only in dividing cells and exert lysis and antioncogene effect in tumor cells without affecting resting normal cells. 5. Myxo- and paramyxoviruses (and other viruses) upgrade the immunogenicity of cell surface antigens thus eliciting rejection type host immunity against these cells which is operational against not virus-infected cells of the same type (post-oncolytic antitumor immunity). 6. Viruses or virally infected cells (including tumor cells) induce the production of lymphokines and cytokines (interferons, interleukins and tumor necrosis factor) and activate NK cells and specific immune T cells cytotoxic to virus-infected cells (including tumor cells). 7. Measles virus may activate suppressor cells and both directly (by infecting lymphoma cells) and indirectly (by inducing molecular mediators of suppressor mononuclear cells inhibitory to the growth of neoplastic lymphoid and hematopoietic cells) induce remissions of lympho- and hematopoietic malignancies. 8. Retroviral vectors deliver genes into tumor cells for encoding new surface antigens that render the tumor cells highly antigenic and more vulnerable to rejection type immune reactions of the host. Examples illustrate each statement. Immunotherapy of tumors with active tumor-specific immunization after the induction of suppressor cells by fetal antigens and the elimination of the proliferating suppressor clones by cyclophosphamide will again be proposed.