Glutathione S-transferases play a major role in the protection of tissues from the toxic effects of exnobiotics and the products of lipid peroxidation. In the present studies we demonstrate that human lung has two forms of glutathione (GSH) S-transferase having isoelectric pH of 4.9 and 9.2. These anionic and cationic forms represent about 98% and 2% of the total GSH S-transferase activity towards 1-chloro-2,4-dinitrobenzene (CDNB). Although the cationic form accounts for only about 2% of the total GSH S-transferase activity of human lung, it comprises 28% of the total GSH S-transferase protein of lung. Both these enzymes express highest activity with CDNB and have similar Km values with this substrate, but the anionic enzyme is more active towards a wider range of substrates. Only the cationic enzyme expresses glutathione peroxidase II activity. Anionic and cationic human lung GSH S-transferases are homodimers of about Mr 22,000 and 24,000 subunits, respectively. Antiserum raised against the anionic GSH S-transferase does not cross react with the cationic form, but does cross react with anionic GSH S-transferases of human placenta, liver, and erythrocytes. A close structural interrelationship between the lung anionic form and placental GSH S-transferases is indicated by similar immunological characteristics and the presence of the same N-terminals for both enzymes.