This study has shown that fresh amniotic epithelial cells which are negative by indirect immunofluorescence for HLA and Trf receptors become positive for both of these markers following prolonged culture. In addition, the amniotic antigens which are characteristic of fresh amniotic epithelial cells are lost and replaced by trophoblast antigens subsequent to maintenance of the cells in vitro. These observations suggest that, although the expression of HLA and Trf receptors are apparently negative in vivo, such gene products become activated and manifest in plasma membranes following exposure of the cells to culture conditions. In addition, the normal manifestation of the amnion antigens is lost following prolonged periods of culture. These findings indicate that the coding and expression of normally accepted transplantation antigens undergo a significant perturbation in vitro, and that markers of normal extra-embryonic epithelium in vivo are lost after the cells have been cultured in vitro.