An effective immunologic destruction of tumors does not take place in most cases, despite the fact that many cancers display antigenic properties and, generally, the host immune system is affected by a complex functional defect. One of the postulated mechanisms of immune suppression in cancer patients is the presence, in body fluids, of soluble factors with inhibitory activity on immune functions. Many different substances have been claimed as soluble immunosuppressor factors in cancer, and most of them have been incompletely characterized. Soluble immunosuppressor factors could be roughly classified according to their origin, if determined (the tumor itself or the host cells) by their mode of action (tumor specific or non-specific suppressors) and nature.