Over the last 15 years, cell transplantation into seminiferous tubules has become a valuable tool to study germinal cell biology and related matters. This is particularly so, because the blood-testis permeability barrier establishes a sealed compartment which protect against certain influences such as immunological rejection. In the light of the functional and genetic similarities between carcinoma in situ (CIS) of the testis and embryonic stem (ES) cells, our laboratory has developed a tumor assay to study cancer invasion processes in testicular germ cell tumors (TGCT) based on the transplantation of ES cells into the seminiferous tubules. Here, we describe this new tumor assay and provide additional information regarding the transplantation techniques used and their application for the study of TGCTs. Finally, we discuss the practical implications of our experimental approach and its potential application for the understanding of TGCT invasive processes and the development of new antineoplastic strategies.