The expression and organization of epidermal growth factor (EGF) receptor gene in cultured human lung cancer cell lines (5 adenocarcinomas, 3 squamous cell carcinomas, 2 small cell carcinomas and 1 large cell carcinoma) have been studied. Two (PC-8 and PC-9) of the adenocarcinomas overproduced EGF receptor mRNA and protein, and exhibited gene amplification, the magnitude of which was comparable to that of A431 cells. Six cell lines (3 adenocarcinomas, 2 squamous cell carcinomas and 1 small cell carcinoma) expressed EGF receptor gene and its product to a significant level without gene amplification, and the other three cell lines were found to be negative as regards expression.