Classical antineoplastic therapeutic modalities such as surgery, radiation, and chemotherapy not only fail to cure the great majority of neoplasms, but their employment often leads to severe and debilitating side effects associated with severe neoplasm-related morbidity. Immunotherapy as a fourth modality of anti-cancer therapy has already been proven to be quite effective. The astonishing immunophenotypic (IP) heterogeneity of neoplastic cells, the different cytotoxic activity associated with the moiety linked to given monoclonal antibodies (mAb), and mostly the impressive genetic modulation capabilities of cancer cells still remain as yet unsolved difficulties in the present immunotherapy of human neoplasms. The advances in mAb production have revitalised the initial concept of use of cancer cell specific "magic bullets." Antibodies represent new approaches to anti-cancer therapy: they are neoplastic cell-specific and lethal to neoplastically transformed cells via immune effector mechanisms with no toxicity to normal tissues. They are being observed and developed, adhering to the old prayer: "Destroy the diseased tissues, preserve the normal." Strategies for the employment of antibodies include: 1) immune reaction directed destruction of neoplastic cells; 2) interference with the growth and differentiation of malignant cells; 3) antigen epitope directed transport of anti-cancer agents to neoplastic cells; 4) anti-idiotype tumour vaccines; and 5) development of engineered (humanized) mouse mAbs for anticancer therapy. In addition, a variety of agents (e.g. toxins, radionuclides, chemotherapeutic drugs) have been conjugated to mouse and human mAbs for selective delivery to neoplastic cells.