Paraffin-embedded tissue sections from 105 cases of human lung cancer were stained for four isozymes of GSTs by immunohistochemical PAP technique. Of the 16 cases of small cell lung cancer (SCLC) examined 14 were negative for all GST individuals, whereas one undifferentiated squamous cell type and one oat cell type treated with chemotherapy before operation were positive for GST-pi. The total positive rates of GST-pi, GSTs and GST-mu in 89 cases of non-SCLC were 75.3%, 13.5% and 9.7% separatively. Among them squamous cell carcinoma were stained positively for GST-pi in 93.5%, GSTs in 9.7% and GST-mu in 6.5%, while adenocarcinoma were in 69.7%, 15.2% and 6.1% respectively. The expression of GST-pi was weakened corresponding with the decreased degree of differentiation of cancer cell. GST-alpha was not detectable in all specimens studied. Ultrastructure location of GST-pi sites in 11 cases of non-SCLC was detected mainly on lysozymes and heterochromatin in cancer cell by transmission electronic microscopy utilizing colloidal gold labelled anti-GST-pi antibody. These results suggest that GST-pi may be an useful marker for differential diagnosis in histopathology and intrinsic sensitivity to anticancer drugs of lung cancer. GST-mu was expressed in some types of lung cancer with low positive rate and its usage as a marker needs further investigation.