Abstract: A DNA probe has been isolated which is capable of hybridizing to an oligonucleotide sequence coding for a polypeptide from a major 64 Kilodalton protein of human cytomegalovirus (HCMVgp64). The probe has a sequence of at least seventeen (17) to as many as seven hundred twenty-one (721) nucleotides. The probe may be labelled as by radioactivity. The probe has been used to screen DNA fragments constituting a subgenomic library of human cytomegalovirus DNA to obtain DNA fragments coding for the major late protein of human cytomegalovirus. The DNA fragments coding for the major late protein of human cytomegalovirus (HCMVgp64) may be hybridized to DNA fragments of HCMV DNA from an individual having human cytomegalovirus infection. The viral DNA can be used as whole HCMV DNA or as fragments formed by digesting the human cytomegalovirus DNA with a restriction endonuclease such as one of the restriction endonucleases EcoRI, BamHI, XbaI, HindIII and PrtI.