Fourteen tumors of the nasopharyngeal region were analyzed for the presence of Epstein-Barr virus-specific DNA by DNA-cRNA hybridization. These data were compared to the histology of the respective tumors and the seroreactivity of the tumor-bearing patient against EBV-related antigens. With one exception, all tumor pieces containing nasopharyngeal carcinoma cells hybridized significantly with EBV-cRNA. Tumors of predominantly epithelial morphology annealed in the highest range. In situ-hybridizations of freeze sections from a tumor containing almost equal amounts of tumor cells and lymphocytes revealed hybridizing DNA within nuclei of non-lymphoid cells. Although these data do not exclude the presence of EBV-DNA within lymphoid cells, they clearly demonstrate that in nasopharyngeal carcinomas the vast majority of EBV-specific DNA rests within non-lymphoid cells.