Insulin-like growth factors are potent mitogenic factors in human lung cancer in vitro, acting via specific receptors. Using monoclonal antibodies we demonstrate the expression of insulin-like growth factor receptor I in bronchial epithelial cells of normal lung and in primary lung cancer (22/24 cases), being most prominent in squamous cell carcinoma. Electron microscopy on lung cancer cell lines reveals a distinct reaction pattern on the plasma membrane. Immunoreaction with a specific antibody directed against the insulin-like growth factor receptor II suggests a weak expression in primary lung cancer. Our findings underline the significance of the autocrine pathway of insulin-like growth factors in lung cancer.