Progressive multifocal leukoencephalopathy and malignant lymphoma of the brain were noted at postmortem examination in a 68-year-old white woman who was treated with immunosuppressive agents after renal transplantation. The two diseases are not uncommon in patients with immunodeficiency, but their occurrence in the same patient is extremely rare. This association suggests the oncogenicity of papova viruses in man. However, no papovavirus was demonstrated in the tumor by electron microscopy and immunohistochemical staining. The immunohistochemical staining of routine histology sections for the common antigen of polyomaviruses by the peroxidase anti-peroxidase technique is shown to be simple and specific for the detection of polyomaviruses in the demyelinated areas of progressive multifocal leukoencephalopathy.