A SV40-like virus was isolated from the brain of a patient with progressive multifocal leukoencephalopathy. The virus was antigenically and serologically indistinguishable from SV40 wild type. A unique difference from SV40 was its ability to grow on human glial and CV-I monkey cells. The molecular weight of the viral DNA was very similar to that of SV40 DNA and the DNA cleavage patterns obtained after digestion with Hind II and Hind III restriction endonucleases were indistinguishable from those of SV40.