Disruption of the two thioredoxin genes in yeast dramatically affects cell viability and growth. Expression of Arabidopsis thioredoxin AtTRX3 in the Saccharomyces thioredoxin Delta strain EMY63 restores a wild-type cell cycle, the ability to grow on methionine sulfoxide, and H2O2 tolerance. In order to isolate thioredoxin targets related to these phenotypes, we prepared a C35S (Escherichia coli numbering) thioredoxin mutant to stabilize the intermediate disulfide bridged complex and we added a polyhistidine N-terminal extension in order to purify the complex rapidly. Expression of this mutant thioredoxin in the wild-type yeast induces a reduced tolerance to H2O2, but only limited change in the cell cycle and no change in methionine sulfoxide utilization. Expression in the Delta thioredoxin strain EMY63 allowed us to isolate a complex of the thioredoxin with YLR109, an abundant yeast protein related to PMP20, a peroxisomal protein of Candida. No function has so far been attributed to this protein or to the other numerous homologues described in plants, animals, fungi, and prokaryotes. On the basis of the complementation and of low similarity with peroxiredoxins, we produced YLR109 and one of its Arabidopsis homologues in E. coli to test their peroxiredoxins activity. We demonstrate that both recombinant proteins present a thioredoxin-dependent peroxidase activity in vitro. The possible functions of this new peroxiredoxin family are discussed.