We have isolated cDNAs for three class I genes from an 8.5-day C57Bl/6 mouse embryo cDNA library. Two of these cDNAs encode the classical transplantation antigens, H-2Kb and H-2Db. The third is a novelly spliced form of a Qa region gene, Q9, which lacks several exons including that encoding the transmembrane domain. Since this mRNA is detected on membrane-bound polysomes it may encode a secreted protein. Transcripts for these three genes and the pseudo-allele of Q9, Q7, were present at very low levels in the 8.5-day and 14.5-day embryo and in the 2-day neonate, compared to most adult tissues. Q10, the liver-specific secreted class I gene product in adults, is preferentially expressed in yolk sac and in fetal liver. We found no embryo-specific class I gene expression.