By employing an ELISA for detection of glutathione S-transferase-pi (GST-pi) established in our laboratory, gel filtration profiles of GST-pi in the plasma of normal subjects and patients with malignant tumors were investigated. The results showed that the plasma GST-pi for both of these groups was approximately half the molecular size of placental GST-pi used as a standard control. Similar analyses were performed on GST-pi of platelets and cultured cancer cells, which are considered to be the main sources of the GST-pi in the plasma of normal subjects and cancer patients, respectively. The results indicated that the GST-pi in both the centrifuged supernatants of aggregated platelets and in the culture medium of cancer cells was about half of the molecular size on intact GST-pi. Moreover, the GST-pi in the culture medium was shown to have an N-terminus and a C-terminus, by analysis with specific ELISA. Western blot analysis of the GST-pi in the culture medium detected a single band migrating at 23 kDa, confirming that the extracellular GST-pi was the monomer, not a cleaved form of intact GST-pi. The release of GST-pi from cancer cells was suppressed at 4 degrees C, or by sodium azide, but not suppressed by colchicine or cytochalasin B. These findings suggest that GST-pi may be released by an energy-dependent, active process, and not by a secretion mechanism.