Opportunistic viral infections in immunosurpressed patients may morphologically involve only a few cells. Identifying an optimal area by LM is most important before an attempt at detection of viral particles by EM. The purpose of this communication is to emphasize unusual and important structural findings for some viruses.

BKV - BK polyomavirus latently infects renal tubular epithelial cells. Immunosuppressed patients many develop reactivation with resultant tubular cell damage and acute interstitial nephritis. Ultrastructural examination shows spherical and tubular virus particles of typical polyomavirus. Cross sections of the tubular form of this virus may contain electron dense cores (fig.l). Cells may be coinfected with HIV on rare occasions (fig.2).

JCV - JC polyomavirus is the major cause of progressive multifocal leukoencephalopathy (PML). JCV are rarely seen in reactive bizarre astrocytes. Biopsy of the tissue at the periphery of lesions of PML is likely to include JC-infected oligodendrocytes; it is important to appreciate that astrocytes, regardless of appearance, are unlikely to contain the virus.