Many tumor markers have been described in the hope of finding a blood test for cancer, and some have found their way into widespread but indiscriminate clinical use. Classically, a marker is synthesized by the tumor and released into the circulation, but it may be produced by normal tissue in response to invasion by cancer cells. Several characteristics define the ideal tumor marker: It should be produced by the tumor cell and be readily detectable in body fluids, not present in health or benign disease, reflect the bulk of the disease and correlate with the results of anticancer therapy. No marker described to date meets all of these criteria. Application of monoclonal antibodies to detect tumor markers promises to greatly expand the number of diagnostic tests available. Monoclonal antibodies have found to provide a degree of specificity not possible using conventional heterogeneous population of antibodies. Antibodies to tissue specific markers are being used to label tissue sections by immune labelling techniques such as immunofluorescence and immunoperoxidase. Identification of a particular marker in the tumor cells of a metastatic lesion may aid in determining the primary site of a tumor. Furthermore radiolabelled antibodies have been used to localize antigen-bearing tumors. Some occult neoplasms have been localized by this procedure, however the practical usefulness remains to be determined. There is still a need for new markers, specific for certain tumoral tissues. Perhaps by aid of monoclonal antibodies it will be possible to identify molecules having a functional significance for tumor-growth (e.g. oncogene products). Such markers may circulate in ultra small amounts, but may still have significance for the monitoring of patients. The continuing improvements of tumor-associated markers may help to approach in cancer treatment and diagnosis.