Interleukin 4 (IL-4) receptors were detected by a monoclonal antibody on tumor cells of 10 of 29 squamous cell carcinomas and 6 of 17 adenocarcinomas of the lung. None of the small cell carcinomas or carcinoid tumors stained. Parallel sections stained for epidermal growth factor receptors showed that all but 2 of the IL-4 receptor-positive tumors also expressed epidermal growth factor receptors. Positive labeling for IL-4 receptors was also obtained on nonneoplastic bronchial epithelium and on lymphocytes and macrophages infiltrating the tumor stroma. The role of IL-4 and its receptor in normal human lung is unknown, but the expression of IL-4 receptors on particular subtypes of lung tumors suggests that they may have a role in differentiation or proliferation of squamous and adenocarcinomas.