We immunohistochemically investigated the expression of glutathione S transferase π (GST-π) and clarified the correlation between GST-π and the results of chemosensitivity testing on the tissue of primary human squamous-cell carcinoma of the lung. The expression of GST-π was evaluated in 105 cases and their level of chemosensitivity was estimated by the in vitro succinate dehydrogenase inhibition test for cisplatin, Adriamycin, cyclophosphamide, mitomycin C, vindesine and flurouracil. Tumors in which 25% and more of the cells stained for GST-π were classified as having a high GST-π expression, while those tumors demonstrating less than 25% of the cells staining for GST-π were considered to have a low expression GST-π. The percentage of high GST-π was 52% (53 of 105) while that of low GST- π was 48% (52 of 105). No significant correlation between the expression of GST-π and clinico-pathologic factors was observed, while no significant difference in the survival of the two groups was found either. An increase in succinate dehydrogenase activity was recognized in the high-GST-π group compared with the low-GST-π group for each anticancer drug; however, no statistical significance was seen except for cisplatin. In the cases with adjuvant combination chemotherapy using cisplatin after a complete resection, all cases demonstrating a relapse were associated with a high GST-π. These findings thus indicate that an over-expression of GST-π is related to the resistance to cisplatin in human lung squamous-cell carcinoma. It is therefore important to select carefully the optimal anticancer drug for hi