An accurate and precise evaluation of tumor-host relations is an essential prerequisite for a rational approach to cancer therapy, in particular immunotherapy, and to a correct assessment of prognosis. The host–tumor relationship is assessed mainly by the capacity of immune components, such as lymphocytes, macrophages, or antibodies, to react in vitro against tumor cells. It is noted that the expression of immunity is not equal in all sites of the body. It is significant to evaluate in situ tumor immunity concerning the immune-histology of the malignant area in relation to prognosis or treatment. This chapter summarizes the available data on the presence, properties, and functions of humoral immune components, mainly immunoglobulins, at the site of malignant tumors. It focuses on the studies that deal with nonlymphoid malignancies. Common methods used to detect tumor-associated immunoglobulins (TAIg) include (1) the treatment of tumor fragments with low pH buffers or with salt solutions of high molarity, (2) direct membrane immunofluorescence of tumor cells, (3) radioimmunofixation, and (4) mixed hemadsorption. The chapter considers several TAIg such as Ig class and IgG subclass.