Fifty-four human lung tumours have been immunostained with a large panel of monoclonal antibodies including reagents against cytokeratins, prekeratins, epithelial membrane antigen (EMA), carcinoembryonic antigen (CEA) and neural antigens. These results have been compared with the histological types of tumour using the current WHO classification scheme. The most striking finding of this study was the considerable overlap of antigenic profile between different histological types of tumour. This suggests that there may be a greater underlying similarity between different histological categories of lung tumour than has hitherto been assumed. Secondly it was evident that immunostaining highlighted areas of different morphology within many tumours emphasizing the heterogeneous differentiation patterns seen in lung cancers. The present study supports the viewpoint that lung tumours arise from a common stem cell and that these neoplasms represent a single tumour with a tendency to differentiate along one or more pathways.