Using glutathione affinity chromatography followed by isoelectrofocusing, we purified from the skin secretion of Xenopus laevis an isoenzyme of glutathione S-transferase with an apparent subunit molecular mass of 22.5 kDa and an isoelectric point at pH 5.1. Its N-terminal amino acid sequence was highly similar to that of the sigma class glutathione S-transferase, which previously was demonstrated to have a glutathione-dependent prostaglandin D2 synthase activity. Immunohistochemistry analysis revealed that the isoenzyme was located in the cytoplasm of granular gland cells.