Spermatogenesis is dependent on a small population of stem cells. Despite the biological significance of spermatogonial stem cells, their analysis has been hampered by their scarcity. However, spermatogonial stem cells can be enriched by selection with an antibody against cell-surface molecules. In this investigation, we searched for new antigens expressed on spermatogonial stem cells. Using the spermatogonial transplantation technique, we examined expression of the CD9 molecule, which is commonly expressed on stem cells of other tissues. Selection of both mouse and rat testis cells with anti-CD9 antibody resulted in 5- to 7-fold enrichment of spermatogonial stem cells from intact testis cells, indicating that CD9 is commonly expressed on spermatogonial stem cells of both species. Therefore, CD9 may be involved in the common machinery in stem cells of many self-renewing tissues, and the identification of a common surface antigen on spermatogonial stem cells of different species has important implications for the development of a technique to enrich stem cells from other mammalian species.