Immunoselection has been used to identify human lymphocytes that have undergone spontaneous mutation resulting in the loss of expression of one of the codominant HLA-A alleles. Approximately 35% of such mutations are the consequence of mitotic recombination events. Mitotic recombination is the result of nonsister chromatid exchange that leaves the mutated cell homozygous for all loci distal to the crossover point. The location of the crossover has been regionalized for 99 independently derived mutant lymphocyte clones by identification of their loss or retention of heterozygosity at seven reference polymorphic loci on chromosome 6. If a polymorphic locus of unknown map position is studied in clones from this ordered set of mitotic recombinants, and clones that display loss of heterozygosity and retention of heterozygosity of the locus are observed, then the map position of the locus is between the appropriate reference loci of the ordered set. The newly mapped locus becomes a reference locus in turn. In this way the mitotic recombinant mutant clones can be used to generate an ordered set of crossover points with a theoretical resolution limited only by the number of mutants generated. In this paper such a set of mutants is used to refine or confirm the map position of eight polymorphic loci on chromosome 6.