We have assessed the diagnostic efficacy of a novel polyclonal rabbit antiserum directed to the recombinant major capsid protein VP1 of JC virus (JCV). Immunohistochemistry for VP1 was compared to non-radioactive JCV DNA in situ hybridization (ISH) in ten cases of progressive multifocal leukoencephalopathy (PML). Tissue sections from postmortem brains were studied from PML patients suffering from immunodeficient conditions of various causes: immunodeficiency syndrome (AIDS, n = 7), severe combined immune deficiency due to adenosine deaminase deficiency (n = 1), sarcoidosis (n = 1) and leukemia (n = 1). VP1 immunohistochemistry demonstrated the presence of JCV in lesional oligodendrocytes of all PML patients, whereas ISH was able to detect JCV in nine out of ten cases. We conclude that VP1 immunohistochemistry is a specific, sensitive and rapid method for confirming the diagnosis of PML.