Senescence-Accelerated Mouse (SAM) is a murine model of accelerated senescence, which consists of the senescence-prone P series and the senescence-resistant R series of strains. In order to characterize these SAM strains molecular genetically, we have performed a series of Southern hybridization experiments using oligonucleotide probes designed to recognize the endogenous mouse retrovirus sequences. The repertoires of endogenous retroviruses in different SAM strains indicated that each SAM strain is distinct. Comparisons of the SAM strains with the parental AKR/J strain revealed significant differences between them, suggesting the involvement of other strains in the course of the development of SAM. While some of the endogenous retroviruses were found in all of the SAM strains, others were found to be distributed uniquely, indicating their potential usefulness as genetic markers in the analysis of strain-specific phenotypes, and possibly of the phenomenon of accelerated senescence itself.