A radioimmunoassay procedure for the quantitation of glutathione transferase-π was developed in order to determine the levels of this protein in human urine. The enzyme was isolated from human placenta with a purification factor of 366 (compared to the original high-speed supernatant fraction), and upon gel electrophoresis, only a single band was seen. Polyclonal antisera were subsequently raised in rabbits and found to be suitable for a radioimmunoassay. Glutathione transferase-π was localized immunohistochemically to the cells of the distal tubules, the thin loop of Henle and the collecting ducts in the kidney. In contrast, the α-isoenzyme was localized exclusively in the proximal tubular epithelium. Samples of urine from healthy individuals contained about 6 ng of the π-transferase/ml. The method proved to be specific for glutathione transferase-π, and no cross-reaction with the α- or μ-transferases or with other proteins occasionally appearing in urine occurred. The protein was quite stable upon storage and insensitive to variations in the urine pH. Thus, it appears that glutathione transferase-π can be conveniently quantitated by radioimmunoassay and changes in the concentration of this protein in human urine thus mo