The major histocompatibility complex (MHC) class II-like molecules, H2-M, have an essential role in processing and presentation of antigens by the MHC class II molecules, because functional inactivation of these genes lead to surface expression of MHC class II molecules devoid of associated foreign peptides. We have used in situ hybridization to examine the expression of MHC class II and H2-Mb genes in embryonic and neonatal mice and show here that expression of H2-Ab and H2-Mb mRNA is absent in 13.5-day-old mice. However, mRNA for both genes could be detected in the thymus of 14.5- and 15.5-day-old embryos, and the patterns of hybridization suggested that the two genes were expressed in the same cells. In spleen and thymus of neonatal mice the H2-Ab and H2-Mb genes were also coexpressed, with expression being localized to the white pulp of the spleen and to the thymic medulla, which are rich in antigen-presenting cells. The steady-state levels of H2-Ab mRNA appeared to be approximately 10 to 14 times greater that the level of H2-Mb mRNA molecules, irrespectively of the tissue and age, reflecting the different functions of the two molecules.