AbstractThe purpose of this study was to determine whether the detection of cytomegalovirus (CMV) in formalin fixed, paraffin embedded tissue is best accomplished by immunohistology or in situ hybridization. Most of the tissues analyzed had the characteristic cellular inclusions. Thus, the detection rate for the mmunohistochemical assay was markedly dependent on the type of proteinase and varied from 25% with trypsin digestion to 90% with proteinase K digestion. All 16 tissues were positive with the in situ hybridization test, and the number of positive cells in serial sections was slightly greater than the immunohistochemical assay with proteinase K digestion. Thus, the detection rates of CMV in fixed tissue sections for in situ hybridization and immunohistochemistry were equivalent if proteinase K digestion was used for the latter technique. (The J Histotechnol16:27, 1993)