A retrovirus element (TLev1) is located within the Thymus leukemia antigen (Tla) locus of the C57BL/10 mouse major histocompatibility complex. Low-copy probes have been isolated from sequences flanking the TLev1 integration site to examine the distribution of TLev1 among inbred mouse strains having genotypically determined variations in TL-antigen expression. It was found that the low-copy probes cross-hybridize to regions within the Tla locus in a genotype-specific manner. Although a strong association was found between TL mouse strains and TLev1, the presence or absence of the TLev1 locus did not exclusively correlate with expression or nonexpression of TL antigens. Analysis of different Mus subspecies indicates that TLev1 integrated into a common ancestor of the species Mus musculus. It is suggested that the loss of the TLev1 locus from certain mouse genomes reflects evolutionary rearrangements in the TL region; the resulting diversity may relate to the differential expression of TL antigens among mouse strains. The probes described here provide a useful tool for examining the genomic expansions and contractions which have occurred during the evolution of the Tla locus.