Cellular studies are essential in the xenotransplantation field in order to investigate the cellular immune responses triggered by xenogeneic cells and identify the key molecules involved. A series of functional studies can be conducted with this purpose that include treatment with proinflammatory cytokines and xenogeneic cell-based assays that put together pig cells and human leukocytes such as monocytes, NK cells, and T cells. The choice of the pig cell type is critical to appropriately model the transplant setting of interest. Thus, pig endothelial cells are commonly used for studying the rejection process of vascularized organs. Treatment with cytokines allows studying the regulation of adhesion, costimulatory molecules, and receptors involved in triggering the immune response in an attempt to reproduce the more complex in vivo situation. The adhesion assays are used to determine the capacity of human leukocytes to adhere to porcine cells under various conditions. Furthermore, we describe coculture, costimulatory, and cytotoxicity assays for investigating the cellular and molecular mechanisms that take place during the xenogeneic immune response.