All polyomaviruses have a common antigen, which is located on the major capsid polypeptide (VPI). This antigen was demonstrated in routine histologic sections of formalin-fixed paraffin-embedded tissues by the peroxidase-antiperoxidase technic. Antiserum to the common determinant reacted with JC virus in oligodendrocytes of a patient who had progressive multifocal leukoencephalopathy, BK virus in bladder epithelium of a renal transplant patient, and K virus in endothelial cells of mouse lung. This method may be applicable to the screening of tissues for all polyomaviruses in diagnostic and investigative pathology.