Immunohistochemical demonstration of the placental form of glutathione S-transferase, a detoxifying enzyme in human gliomas. Expression of the human placental form of glutathion S-transferase (GST-pi) in human gliomas was investigated by immunohistochemical methods, and the result was compared with that of normal human glial cells. The gliomas in this study were composed of five benign astrocytomas (Grade 2), ten anaplastic astrocytomas (Grade 3), and 16 glioblastomas (Grade 4). Normal human glial cells showed only a weak immunostaining response for GST-pi in the cytoplasm or some nuclear membranes. All of benign astrocytomas had diffusely weak GST-pi immunostaining, resembling that of normal glial cells. With increasing grade, gliomas showed a strongly positive reaction for GST-pi. The positive reactions were remarkable especially in the gemistocytes and giant cells in the high-grade gliomas. These results suggest that cells of gliomas have some detoxifying function and the expression of this detoxifying enzyme, GST-pi, is related to the degree of malignancy of the gliomas. 