Thioredoxin is a powerful redox protein expressed in invasive cytotrophoblasts and essential for blastocyst implantation in mice. Isolated marmoset thioredoxin cDNA showed that the deduced amino acid sequence differed from the human sequence by four amino acids. The close homology of thioredoxin in the two species enabled us to use monoclonal antibodies against human thioredoxin to detect marmoset thioredoxin in implantation sites, blastocysts and culture medium. Immunocytochemistry on marmoset implantation sites, on pregnancy days 12 and 15, showed that thioredoxin is highly expressed in uterine luminal epithelium, glands and in some endometrial stromal cells. In attached blastocysts, thioredoxin staining was detected in mural and polar trophoblast cells and both visceral and parietal endoderm, whereas no staining was present in the inner cell mass. A similar pattern of thioredoxin expression was detected in hatched blastocysts attached to Matrigel in tissue culture. Trophoblastic vesicles derived from blastocysts expressed thioredoxin in inner endoderm-like cells and outer trophoblast-like cells and secreted thioredoxin into the culture medium. These experiments have demonstrated thioredoxin expression during early stages of embryo-maternal interaction. We propose that thioredoxin protects the early placenta from oxidative damage and that the marmoset is a valuable model for studying thioredoxin regulation and function during implantation and blastocyst differentiation.