AbstractA case of cytomegalovirus (CMV) infection of the bladder is described, with particular reference to the use ofIn Situhybridization as a diagnostic aid. Morphologically, it was not possible to identify characteristic cytomegalic cells in the tissue. Immunocytochemistry using human serum from an infected patient gave equivocal staining due to cross reaction of the labeled anti-human immunoglobulin serum with tissue proteins. A commercially available monoclonal antibody produced clearer results.In Situhybridization with biotin labeled CMV DNA was able to detect virus unambiguously in routinely prepared paraffin embedded tissue sections. The technique is based on the use of a high temperature to denature tissue DNA sequences, followed by simple immunological detection of hybridized biotin residues. We suggest thatIn Situhybridization should be considered as a diagnostically useful tool in the histological investigation of viral infection. (The J Histotechnol12:229, 1989).