Cell surface expression of human class I molecules in transgenic mice is dependent upon the available pool of beta 2-microglobulin (beta 2m) and the affinity between mouse beta 2m and human class I molecules. HLA-B27 and HLA-Cw3 transgenes can be expressed in mouse strains of the H-2 haplotypes b, f, k, and s which encode two endogenous class I genes mapping to H-2K and H-2D. The human class I genes cannot be expressed on H-2d and H-2q haplotypes which encode three endogenous class I molecules (K,D,L). This suggests that there may be only enough mouse beta 2m molecules to support three class I molecules. When both the HLA-B27 and HLA-Cw3 genes are introduced into H-2b mice, only HLA-Cw3 reaches the cell surface. This suggests that HLA-Cw3 has a higher affinity than HLA-B27 for mouse beta 2m. The possible implications of our findings regarding the assembly, transport, and expression of class I MHC molecules in vivo are discussed.