Abstract Becuase more than 80% of all cancer deaths are caused by metastases, development and evaluation of methods for fighting tumor dissemination should be major tasks of present cancer research. Formation of metastases is favoured by both reduced numbers of immune cells in the bloodstream and impaired oxygen transport into tissues. These closely related signs often emerge concomitantly when the organism is endangered by circulating tumor cells released from the original tumor by therapeutic manipulations. From knowledge of these facts the O 2-multistep immunostimulation technique has been developed as a way of diminishing the risk of tumor spread. The process combines temporary elevation of the number of circulating immune cells with continuous improvement of oxygen transport into tissues. The former is achieved by a peptide mixture isolated from thymus glands in combination with the chemical immunomodulator 2-cyano-ethyl urea; the latter is the outcome of several variants of the O 2-multistep therapy discussed here. The efficiency ranges of the different variants are quantified on the basis of findings that allow assessment of the number of tumor cells which can be destroyed by this treatment. This number may be about 100 times the number of malignant cells that must be killed in terms of an effective metastasis prophylaxis (≈ 3 × 10 5). The estimated efficiency range represents therefore a not yet fully exhausted preventive and possibly even therapeutic potential. — To speed the introduction of the procedures described into practice, all clinical oncologists are encouraged to refer their patients to established facilities for O 2-multistep immunostimulation after termination of any conventional therapy.