Glutathione S-transferases (GSTs) are enzymes that inactivate toxic compounds by conjugation with glutathione and are involved in resistance towards drugs, antibiotics, insecticides and herbicides. Their ability to confer herbicide tolerance in plants provides a tool to control weeds in a wide variety of agronomic crops. GST-III was prepared from Zea mays var. mutin and its amino acid sequence was determined from two sets of peptides obtained by cleavage with endoprotease Asp-N and with trypsin, respectively. Recombinant GST-III was prepared by extraction of mRNA from plant tissue, transcription into cDNA, amplification by PCR and expression. It was crystallized and the crystal structure of the unligated form was determined at 2.2 A resolution. The enzyme forms a GST-typical dimer with one subunit consisting of 220 residues. Each subunit is formed of two distinct domains, an N-terminal domain consisting of a beta-sheet flanked by two helices, and a C-terminal domain, entirely helical. The dimeric molecule is globular with a large cleft between the two subunits. The amino acid sequence of GST-III and its cDNA sequence determined here show differences from sequences published earlier.