Epidermal growth factor (EGF) promotes the growth of cultured benign and malignant cells. Recent studies have demonstrated that the amount of EGF receptor is elevated in squamous cell carcinoma cells in tissue culture when compared with normal epidermal cells. This study demonstrates that elevated levels of EGF receptor are detected in biopsy specimens of human squamous cell carcinomas of the lung with a murine monoclonal antibody, EGF-R1, which binds specifically to the receptor. The increased receptor ranged from 2.5- to 5-fold that of normal skin. These findings have been observed in 11 of 11 squamous carcinomas and two of two epidermoid head and neck cancers. Seven of eight adenocarcinomas, two of two small cell, and four of eight undifferentiated lung cancers had negligible amounts of EGF receptor. The EGF receptor antibody did not bind significantly to normal lung tissues and 35 nonepidermoid tumors. Therefore, EGF receptor may be an excellent marker for epidermoid malignancies.