The major transplantation (MHC) Ag in the mouse are cell-surface glycoproteins composed of a 40- to 45-kDa H chain and a 12-kDa L chain, beta 2-microglobulin (beta 2 m). Regulation of MHC gene expression during early development is thought to play a critical role in maternal tolerance of the fetal allograft. We have used in situ hybridization techniques to characterize the temporal and spatial pattern of expression of the beta 2 m gene in the developing mouse embryo. beta 2 m mRNA is first detected in the extra-embryonic derivatives of the early primitive streak stage embryo where it is specifically restricted to the tissues of the ectoplacental cone and chorion. Expression of the beta 2 m gene continues in these tissues during formation of the definitive placenta. beta 2 m transcripts were also detected in the tissues of the visceral yolk sac starting at 10.5 days. Comparable levels of expression were observed in both the visceral yolk sac endoderm and the extra-embryonic mesoderm. These results clearly indicate that the onset of beta 2 m gene expression is not restricted to early hemopoietic cells. In the embryo-proper, significant levels of beta 2 m mRNA were not detected until day 9.5. At this stage, we observed a strong signal over the liver rudiment. The remaining somatic tissues expressed low levels of transcripts. Similar results were obtained at later stages of development, i.e., the fetal liver was consistently the most strongly positive organ. Overall these in situ hybridization studies define the pattern of beta 2 m gene transcription and thus potential sites of surface expression of class I proteins in specific cell lineages in the early postimplantation stage embryo.