We have studied the expression of both HLA class and the recently described HLA class I-like genes (HLA-E and HLA-6.0) in two human developmental tumor cell lines, that serologically could not be typed for HLA-A, -B, and -C. Evidence is presented that the teratocarcinoma Tera-2 stem cells express trace amounts of non-beta 2 microglobulin-associated classical HLA-A, -B, and -C Ag on their cell surface. The Jeg-3 human choriocarcinoma cells derived from fetal trophoblast express normal levels of surface beta 2 microglobulin associated with 45-kDa and 41-kDa H chains that display unique isoelectric points in IEF gels. The HLA-6.0 or HLA-E genes may encode the lower molecular mass component on Jeg-3 cells, because these genes express molecules of about 38 kDa and 41 kDa, respectively, when transfected to appropriate host cells. Transcripts homologous to the HLA-6.0 gene are found to be constitutively expressed in the Jeg-3 cells but not in Tera-2 cells, whereas HLA-E is not significantly transcribed in either cell line. Transcription of class I (HLA-A, -B, and -C) and class I-like (HLA-E and HLA-6.0) genes is up-regulated after treatment with IFN-gamma in Tera-2 cells, whereas the corresponding genes in Jeg-3 cells remain essentially unresponsive. The biologic role of trophoblast class I(-like) molecules in the context of the maternal acceptance of the fetal semiallograft is discussed.