The glutathione S-transferases are a group of enzymes involved in the detoxification of a wide range of xenobiotics. Elevation of the level of activity of glutathione S-transferases within the cytosol has been associated with the development of resistance to a number of cytotoxic drugs, including some commonly used in the treatment of leukaemia. In this paper we describe the purification and characterization of an anionic (p class) form of the enzyme from the peripheral blood of patients with acute myeloid leukemia, chronic myeloid leukaemia, and acute lymphocytic leukaemia and the spleen of a patient with chronic lymphocytic leukaemia. We present evidence that the form of enzyme purified closely resembles pi class glutathione S-transferase purified from human placenta. Immunoblotting performed on cytosol from the leukaemic cells from a range of cases of leukaemia at presentation, or on treatment, demonstrated that this form of glutathione S-transferase was the predominant isoenzyme expressed in all cases studied. However, in the limited number of cases studied there was no correlation between the level of expression and response to chemotherapy, suggesting that increased expression of pi class GST is not the sole cause of resistance to bifunctional alkylating agent in human leukaemias.