Mouse and human ES cells express high levels of membrane alkaline phosphatase (AP) and Oct-4, a transcriptional factor critical to ICM and germline formation. However, unlike mouse ES cells, human ES cells do not express stage-specific embryonic antigen (SSEA-1). In addition, prolonged propagation of hES cells is typically achieved by co-culture with primary mouse embryonic fibroblasts (MEFs) serving as feeder cells. Although stem cells from different origins require different growth conditions for self-renewal and display different cell surface markers, AP is the most widely used stem cell marker. The StemTAG™ Alkaline Phosphatase Activity Assay Kit provides an efficient system for monitoring ES cell undifferentiation/ differentiation through AP activity in a standard microplate reader. Each kit is sufficient for 100 assays.