We characterized murine spermatogonial stem cells (SSCs) using a multi-parameter selection strategy, combining Oct4 expression determined by monitoring green fluorescent protein (GFP) expression, and the testicular side population (SP) showing weak fluorescence on Hoechst 33342 dye staining, as markers of stem cell purification. Testicular cells were collected from Oct4/GFP transgenic mice and analyzed using a fluorescence-activated cell sorter (FACS). SP was detected in testicular cell suspensions at an average rate of 0.10%. Multicolor analysis indicated that 96% of SP cells were negative for Oct4. The cells did not express SSC marker genes, but expressed Bcrp1. While the main population was 93% positive for pyronin Y staining, this was limited to 51% in SP. We found a novel subpopulation with reduced RNA content lacking Oct4 expression in testicular SP. These results suggest that the cells isolated by FACS represent a novel population of SSCs in the G0 quiescent state.