beta 2‐Microglobulin is the smaller, relatively non‐polymorphic chain of class I major histocompatibility complex proteins. We have previously described a mutant mouse cell line which had been selected for loss of the class I thymus leukemia (TL) antigen and had concomitantly lost surface expression of H‐2k antigens. Expression of class I antigens on the cell surface was restored by fusion to an antigenically distinct mouse lymphoma line, and the defect in the mutant was shown to be the loss of a functional beta 2‐microglobulin gene. We now describe three additional mutants with the same phenotype, all selected for loss of TL but after different types of mutagenesis. All of these mutants have genomic rearrangements resulting in the absence of a functional beta 2‐microglobulin gene. These data provide strong evidence for the requirement of beta 2‐microglobulin for cell surface expression of the heavy chain of class I major histocompatibility complex proteins. We further show that the defects in at least one beta 2‐microglobulin gene in each mutant cell line map to the same small DNA segment within the first intron. The breakpoints of these mutations define a hypermutable site within the mouse beta 2‐microglobulin gene.