Primary hepatocytes isolated from human, mouse and rat liver as well as cell lines, such as HepG2 cells, are frequently used in vitro systems in liver research. In regenerative medicine, stem cells are used for differentiation towards hepatocyte-like cells with the goal of creating differentiated functional hepatocytes. It is therefore important to measure the quality and function of highly specialized hepatocyte-specific functions using appropriate methods. In this chapter, we describe an assay to assess conjugation and excretion of labeled bile acids in cultured hepatocytes.