To determine whether variations in the expression of tumour related antigen can predict the origin of tumours, the immunoreactivity of a series of adenocarcinomas from a wide range of sites was studied with a panel of monoclonal antibodies with specificity for carcinoembryonic antigen, carcinoembryonic antigen with non-specific cross reacting antigen co-specificity, epithelial membrane antigen, OC 125, and OC 19.9. A range of reactivity was seen in tumours from most sites. No distinctive results were identified for a particular site of origin. Some patterns of consistent positivity or negativity for the panel of antisera used were seen, however, which if applied to metastatic deposits have the potential to assist prediction of the site of origin. These results also imply that immunohistological reactivity should be considered in the selection of a tumour marker for serological monitoring.