The Gpxh gene from Chlamydomonas reinhardtii has been reported to code for a GPX homologous protein. The classical function of GPXs is to catalyse the reduction of hydroperoxides using glutathione as electron donor and are thus involved in the protection of cells against oxidative damage. In order to investigate the oxidative stress defence system in photosynthetic organisms, we investigated the function of Gpxh in Chlamydomonas reinhardtii. We thus determined the GPX activity by heterologous expression of Gpxh in Escherichia coli and measuring the enzyme activity of the extract in vitro in a coupled reaction with GSH, GSH reductase and NADPH, monitoring the oxidation of NADPH. We could thus prove activity of Gpxh towards organic hydroperoxides which are comparable to other GPXs in plants and yeast. Complementation experiments with Saccharomyces cerevisiae strains deleted in GPX homologous genes suggest a partial functional homology of Gpxh with GPX3, which is known to be involved in membrane protection and signal transduction. In order to further determine the role of the Gpxh protein, we carried out an in vivo localisation of Gpxh by GFP tagging. The results of the activity measurements and the complementation experiments in this study, together with previous findings, indicate two possible functions of Gpxh, one in the protection of membranes against oxidative damage, and one in redox sensing and -signaling.