Many biochemical indices are purported to have clinical utility in the detection and management of neoplasia. Experience gained during the past decade tends to indicate their having a more important role in the detection and monitoring of metastases than of the primary lesion. From this present review of some of the commoner human tumours, it is concluded that such marker substances are important adjuncts in the management of germ cell and certain endocrine and endocrine-related tumours. The carcinoembryonic antigen (CEA) provides a marker for many gastrointestinal cancers, but there are no presently available substances with clinical usefulness for either breast or lung neoplasms. Alternative approaches to the detection of metastases are also presented. The particular use of antibody probes at an immunohistochemical level has been claimed to be able to detect micrometastastic disease in bone marrow or tumour-related monoclonal antibody probes may have application to other cancers in the future.