TA02 (napsin A) associated with primary lung adenocarcinoma detected by the two-dimensional polyacrylamide gel electrophoresis (2-DE), has a molecular weight of 35.0 kDa and an isoelectric point 5.29. We developed a mouse monoclonal antibody against the N-terminal of the TA02 molecule (clone TMU-Ad02 (6A1)) and attempted to investigate its immunohistochemical reactivity for nodular lesions of the peripheral lung, and evaluate its usefulness for making a differential diagnosis between primary and metastatic lung adenocarcinomas. The TA02-immunohistochemical evaluation was carried out using surgically resected materials of 109 cases of lung cancer (76 cases of primary lung cancer and 33 cases of metastatic lung adenocarcinoma from other organs), five cases with mesothelioma, four cases with atypical adenomatous hyperplasia (AAH) and five cases with tuberculoma of the lung. In 39 out of 43 (90.7%) primary lung adenocarcinomas, positive immunohistochemical reactivity with a granular cytoplasmic staining pattern was observed. Even though two out of seven (28.6%) large cell carcinomas showed positive results with a granular staining pattern, the immunohistochemical staining intensity was weak. The other cancerous lesions containing metastatic lung adenocarcinoma and mesothelioma were all completely negative. In comparison with the immunohistochemical reactivity of TA02 and SpA, TA02 was superior to SpA both in terms of positive rate and immunohistochemical intensity. We concluded that the evaluation of TA02 expression in lung tumorous lesions was very valuable for discriminating between primary and metastatic lung adenocarcinoma, and TA02 was superior to SpA as a discriminating marker.