We have examined 18 primary malignant lung tumours categorized as either carcinosarcoma, blastoma or spindle-cell carcinoma according to accepted criteria. Two monoclonal antibodies to keratins, CAM 5.2 and LP 34, were used to determine whether the non-epithelial or spindle-cell components of each tumour showed evidence of keratin expression. By this means the epithelial nature of the five tumours classified as spindle-cell carcinomas was confirmed. In all four pulmonary blastomas and in five of nine carcinosarcomas, the sarcomatous elements failed to stain for keratin but in the remaining four carcinosarcomas there was focal staining. The histogenesis of these tumours is discussed and it is suggested that the sarcomatous component of a carcinosarcoma may be derived from malignant epithelial cells by a process of mesenchymal metaplasia with a switch in intermediate filament type. It remains uncertain whether blastomas are derived from both endoderm and mesoderm, or from either one of these tissues, with one component representing complete metaplastic transformation.