Human glutathione S-transferase pi was purified from placenta, and when compared to previous procedures a high yield of pure protein was obtained. A specific and sensitive radioimmunoassay suitable for the measurement of human glutathione S-transferase pi in human plasma is described. The sample collection method for glutathione S-transferase pi was found to be a major obstacle in utilising the measurements for diagnostic use, unless a stringent collection protocol was adhered to. A good discrimination between cancer and non-cancer patients was observed, with 66% of lung cancer patients having elevated glutathione S-transferase pi levels with adenocarcinomas showing the most significant rises.