Human amnion mesenchymal cells (AMCs) contain multipotent cells. To enrich such multipotent stem cells, we applied to AMCs the new method for the isolation of side population (SP) cells used for the enrichment of multipotent stem cells from many tissues. We succeeded in obtaining SP cells from AMCs (AMC-SP cells). AMC-SP cells were found in 0.2% of AMCs, irrespective of the length of pregnant period, ranging from 37 to 40 weeks. Cell cycle analyses suggested that AMC-SP cells belonged to a cell population that proliferated very slowly and/or was in a quiescent state in the amniotic membrane. Upon culturing, they proliferated with 40 to 80 cell doublings. However, they did not form colonies in a soft agarose culture, whereas HepG2 cells, representative human hepatoma cells formed many large colonies. These results suggest that AMC-SP cells that have considerable value for the use of regenerative medicine can be managed safely in vitro. We enrich to multipotency cells (AMCs) contain multipotent cells. We succeeded in AMCs the new method for the isolation of side population (SP) cells used for the enrichment of multipotent stem cells from many tissues. Acquisition of SP cells from AMCs (AMC-SP cells). AMC-SP cells were found in 0.2% of AMCs, irrespective of the length of pregnant period, ranging from 37 to 40 weeks. Cell cycle analyzes suggested that AMC-SP cells belonged to After culturing, they proliferated with 40 to 80 cell doublings. However, they did not form colonies in a soft agarose culture, while HepG2 cells, representative These results suggest that AMC-SP cells that have considerable value for the use of regenerative medicine can be managed safely in vitro.