In contrast to other human nucleated cells, amnionic epithelial cells do not normally express class I HLA antigens. It is not known whether the failure of display of those antigens occurs because genes for HLA expression are deleted or are defective, or if gene expression is repressed. The results of the present study demonstrate that amnionic epithelial cells exposed to IFN-gamma express levels of class I HLA antigens and beta 2-microglobulin that are readily detectable in immunoperoxidase and immunofluorescence stains. The findings suggest that failure of transcription may be responsible for failure of normal expression of class I HLA antigens and beta 2-microglobulin by cells of the amnion membrane. Class II antigens, which are normally expressed by very few cell types, were not expressed by amnion cells under any test conditions.