We have studied the prevalence of a 62 kd protein product of the c-myc oncogene in tissue biopsies from 79 primary bronchial carcinomata using the monoclonal antibody Myc 1-9E10 and the avidin-biotin complex (ABC) technique. This oncoprotein was strongly expressed in 43% of 37 squamous lesions, 29% of 14 adenocarcinomata, 42% of 7 non-small cell lesions not further classifiable and 19% of 21 small cell neoplasms, all of classical morphology. There was no statistical difference between groups in the prevalence of its expression, nor was it related to survival. This oncoprotein is commonly expressed in non-small cell as well as small cell bronchial carcinomata and, in the latter, is not confined to those variant tumours which possess a "large cell" morphology and carry a poor prognosis.