Sixty-six lung carcinomas have been examined by light and electron microscopy, as well as by immunocytochemical techniques using a panel of monoclonal antibodies. There was considerable heterogeneity with regard to cell type and in only 18 cases was it possible to classify the tumour as a solely small cell, squamous or adenocarcinoma. In the remaining cases there was evidence of two or three cell types. These findings support the thesis that all lung cancers are derived from a pluripotential basal or reserve cell in the bronchial mucosa which may proliferate along one or more lines of differentiation. This view of the histogenesis of lung cancer would account for the heterogeneous appearance of many tumours and the difficulty experienced in placing them in one of the standard classifications.