Human folate receptor (hFR, folate-binding protein) is a single-chain glycoprotein with high specific affinity for folic acid and methotrexate. We have created 4 monoclonal antibodies (MAbs) to hFR, all of which react specifically with purified hFR in Western blots. Flow cytometry indicated that the antibodies all had patterns of reactivity against epithelial cell lines similar to that of antibody MW207 (workshop antibody 12), labeling 2 breast-tumor cell lines and 2 of 5 SCLC without labeling the one non-small-cell carcinoma tested. We used the antibodies to trace the in situ distribution of hFR in histologically normal tissues and in lung tumors by a sensitive alkaline-phosphatase-anti-alkaline-phosphatase immunohistochemical technique. In frozen sections of normal lung, hFR was diffusely distributed on cell membranes of type-1 and type-2 pneumocytes and mucociliary and basilar respiratory epithelial cells of distal bronchi. The receptor was focally expressed by mucociliary cells of proximal respiratory mucosa and by macrophages, but was not detected in stromal smooth muscle, fibroblasts or lymphoid cells. In tumors, hFR was heavily and diffusely expressed on cell membranes of 9 of 10 pulmonary adenocarcinomas, 5 of 5 bronchioloalveolar carcinomas and 2 of 2 carcinoid tumors. It was focally expressed in 3 of 5 large-cell lung carcinomas and was absent from 4 of 5 small-cell carcinomas, 18 of 22 invasive squamous carcinomas, 2 of 2 in situ squamous carcinomas, and 13 of 13 squamous dysplasias of bronchial mucosa. We conclude that hFR is heavily expressed in situ by normal alveolar and bronchial epithelium and by adenocarcinoma of lung. It is usually absent from small-cell carcinoma and squamous tumors at levels detectable by immunohistochemistry.