Studying human xenografts in mice is a widely used and powerful approach in both, stem cell and cancer research. Since the indispensible role of invaded host cells for teratoma respectively tumor development became evident, sensitive methods to detect mouse infiltration are in demand. For this purpose, in situ-labeling is generally preferred since it allows to draw valuable conclusions on host cell properties such as cell morphology, location and structural assembly. Unlike existing approaches, which employ species-specific antibodies to detect distinct cell types such as endothelial cells, immune cells or fibroblasts, our protocol exploits the species-specific detection of a proven housekeeping protein to visualize the microenvironment as a whole. So far undescribed, this 24-hours fluorescent immunohistochemistry (IHC) protocol enables the detection of mouse cells irrespective of a particular cell type or subpopulation and thus allows to draw are more comprehensive picture of host cell contribution in a single detection step.