A number of the mechanisms implicated in the phenomenon of immunosuppression present in tumour-carrying patients are described. In particular, stress is laid on factors soluble to immunosuppressive activity identified in the serum or in other biological fluids of these patients. From among these are analysed the circulating immunocomplexes, the proteins of the acute phase (alpha-globulins) and other inhibiting substances derived from tumour cells. The nature and origin of these substances are discussed as well as their effects on the cancer patient's immune system and the therapeutic type implications that arise in some cases.