Most testicular germ cell tumors of adults are presumably derived from polyploid carcinoma in situ. Thus, one would expect that even highly differentiated teratoma components are aneuploid and that it is unlikely to find diploid tumor cell (sub)populations. We studied 10 residual mature teratomas (RMTs) using a dual parameter flow cytometry procedure. Nuclear DNA was stained with propidium iodide and cytoplasmic intermediate filament proteins, in particular, cytokeratins, with fluorescein isothiocyanate-labeled specific monoclonal antibodies. Cells in RMTs, immunoreactive with antibodies to cytokeratins were considered to be tumor cells. These were always found to be aneuploid, in agreement with the available cytogenetic data on these tumors. The diploid cells present in RMTs were devoid of cytokeratins; therefore, these cells represent the nonmalignant normal host stromal and inflammatory cells. These results, in accordance with our earlier finding, indicate that diploid testicular germ cell tumors are extremely rare in adults, and that even the histologically benign somatic tissues in RMT after polychemotherapy are aneuploid.