Epidermal growth factor receptor (EGF-R) expression was assessed in 63 lung tumour samples with a monoclonal antibody (EGF-R1) by indirect immunoperoxidase staining on cryostat sections. All 15 small cell lung cancer samples were negative whereas over 80% of the 48 non small lung cancer stained positively. In 30 bronchial biopsies two monoclonal antibodies against the cytoplasmic part of the EGF-R were evaluated. These antibodies showed weaker staining than EGF-R1. No additional or enhanced staining as compared with EGF-R1 was observed, suggesting a lack of enhanced expression of a truncated EGF-R analogous to the v-erb-B oncogene product. Monoclonal antibodies against the EGF-R may be helpful diagnostically in differentiating small cell from non small cell lung cancer and may also be important in elucidating biological differences in primary lung cancer.