First trimester chorionic villi obtained by chorionic villus sampling at approximately 9 weeks of gestation were investigated by indirect immunofluorescence to demonstrate trophoblast cell surface antigen expression. Villous trophoblast expressing the trophoblast specific markers transferrin receptor, human placental lactogen, and cytokeratin was also found to express a monomorphic major histocompatibility complex class I determinant recognized by the monoclonal antibody W6/32. W6/32 positive regions included sparsely scattered regions of villous trophoblast and fanning outgrowths of trophoblast. The class I antigenic determinant expressed by first trimester trophoblast was found to be recognized exclusively by W6/32 when assayed with a panel of anti-class I determinant monoclonal antibodies. Trophoblast W6/32 determinant expression was not increased after 24 hour organ culture in the presence of 200 U of interferon gamma. Exposure to interferon gamma resulted in increased class I antigen expression by mesenchyme and low level de novo mesenchyme class II antigen expression. These data suggest that early gestational stage villous trophoblast express non-classical class I antigens which do not seem to be subject to the regulatory effects of interferon gamma.