The expression of ABH and Lewis antigens has been studied in a series of pulmonary carcinomas, in areas of squamous metaplasia, and in normal adjacent bronchopulmonary tissues by means of a panel of lectins and monoclonal antibodies. All respiratory epithelial cells can express antigens, with the exception of glandular serous cells. The expression of AB antigens is rather homogeneous, while Lewis antigens are expressed in a more irregular pattern, alternating positively stained cells with negatively stained cells in the same microscopic field. The expression of blood group antigens allows the identification of residual pneumocytes inside the tumor and the proper classification of some neoplasms. Metaplastic areas show a variation in the staining profile when compared with normal tissues and pulmonary carcinomas. The most significant findings are the deletion of A antigen and the strong expression of Le antigen. Pulmonary carcinomas are composed by a heterogeneous population and tend to express antigens in the more differentiated cases or areas. The most important findings are the deletion of AB antigens and the strong expression of Le(y) antigen.