An in situ hybrido-immunocytochemical assay, with a digoxigenin-labelled probe, was used to show the presence of cytomegalovirus DNA in both paraffin and frozen sections from tissue blocks of 5 AIDS patients. The hybridization probe was constructed by using two different DNA fragments of the repeated sequences of the CMV genome. The CMV DNA probe hybridized in situ was immunocytochemically visualized by anti-digoxigenin Fab fragments labelled with alkaline phosphatase. This hybridization procedure proved to be sensitive, specific, and provided good resolving power. Thus, it might effectively be employed in immunohistological and virological laboratories for the diagnosis of CMV infections in AIDS patients; indeed it might even be applied further in the virological context.