Approximately 2 500 specimens from 96 renal transplant recipients were examined for virus particles. Electron microscopy of negatively stained preparations showed that only 3 virus groups were present, namely polyomavirus, adenovirus and herpesvirus. No RNA viruses or oncogenic RNA viruses (retraviridae) were detected. Over a 5-year period polyomavirus was excreted by 37,5% of patients. Both herpesvirus and adenovirus were excreted by approximately 11% of the patients. Although trace quantities of other viruses may have been missed, the absence of RNA virus suggests that immunosuppressive treatment activates these DNA viruses. The failure to detect RNA viruses suggests that they are absent or may require different means of activation.