Mutants that fail to express the B2mb allele can be readily isolated from a B2ma/b heterozygous cell line called R8 by a single step immunoselection protocol using a mAb against beta 2 microglobulin protein in the presence of complement. We present data that demonstrate that the mutants preexist the selection and arise at a rate of about 10(-6)/cell per generation. Southern blot analyses suggest that the mutants have lost the B2mb gene and the linked allele at the Emv-15 proviral integration site telomeric to the B2m locus. Surprisingly, the deletion of the first exon of the B2mb gene that is observed in other cell lines is not found in R8. Possible reasons for this observation are discussed.