Epidemiological features suggest that the risk of testicular cancer may be related to exposure to unknown infectious agents, including viruses. Therefore a series of twenty specimens of testicular germ cell tumours, including preinvasive carcinoma in-situ, were tested for the presence of DNA sequences of two viruses with known transforming abilities, human papillomavirus (HPV) and Epstein-Barr virus (EBV). The polymerase chain reaction (PCR) technique was used. In none of the 19 successfully amplified samples were DNA sequences of HPV type 16 or type 18 detected. In six cases a faint trace of EBV DNA was revealed in one of two experiments. These samples were examined by immunohistochemical staining with specific antibodies raised against the EBV protein products and in-situ hybridization with specific molecular probes, and were confirmed to be negative. The study indicates that a significant direct involvement of HPV and EBV in human testicular germ cell carcinogenesis is unlikely. However, a putative growth-stimulating role of EBV-transformed lymphocytes, which are frequently present in the stromal tissues of testicular tumours, cannot be excluded.