Thioredoxin is a redox active protein which has been implicated in reproductive processes. In this study we investigated the intracellular production and extracellular secretion of placental thioredoxin by human cytotrophoblast cell lines which were used as in-vitro model systems. Results clearly demonstrated that thioredoxin is not only synthesized by these cells but is also secreted and that while the intracellular thioredoxin is present only as a 12 kDa form, it would appear that the extracellular thioredoxin is present in two forms, a predominant 12 kDa form accompanied by a lower amount of a 10 kDa form. The observed localization and possible secretion of thioredoxin at the feto-maternal interface suggest important roles for this protein during pregnancy. Intracellular thioredoxin may be involved in the prevention of cellular damage due to oxidative stress whereas extracellular thioredoxin may act to integrate the actions of the cytokine network operating at the feto-maternal interface thereby assisting with implantation and the successful establishment of pregnancy.