Serum albumins of certain animal species (cow, sheep, pig) accelerate the decomposition of prostaglandin endoperoxides, with formation of large amounts of prostaglandin D. The reaction is inhibited by arachidonic acid, which suggests an interaction of the endoperoxide with the fatty acid binding sites of serum albumin. Glutathione-S-transferases, in the presence of glutathione, convert the endoperoxide into a mixture of prostaglandin F2alpha, E2 and D2. The prostaglandin D/E-ratio depends on the transferase used. The known rat liver transferases give mainly prostaglandin F2alpha and E2, but a new transferase in sheep lung was discovered which gives rise to large quantities of prostaglandin F2alpha and D2. The sheep lung transferase was purified to homogeneity. Two iso-enzymes with identical enzymic activity were obtained. The major component (transferase SL 2, an iso-enzyme of glutathione-S-transferase, EC 2.5.1.18) has a molecular weight of 45 000 and consists of two subunits. Its isoelectric point is 9,8-9.9. These properties, as well as the amino acid composition and the substrate specificity for typical transferase substrates, indicate a close resemblance to transferase B (ligandin), a major binding protein of rat liver. Although purified glutathione peroxidase from erythrocytes is very active in catalysing the reduction of the 15-hydroperoxy group of prostaglandins, it does not have any effect on the decomposition of the endoperoxide group.