In this report we analyze the UL104 open reading frame of human cytomegalovirus (HCMV) genome that encodes the putative portal protein. An affinity-purified monospecific antiserum directed against a GST-UL104 fusion protein identified proteins of approximate M(r) 73000 and 145000 in HCMV-infected cells and purified virions. Furthermore, using an in vitro assay the ability of pUL104 to bind double-stranded DNA was shown. Analysis under native conditions of pUL104 revealed that the monomeric and dimeric forms of the protein also form high molecular weight complexes upon sucrose gradient centrifugation. The protein has been purified from recombinant baculovirus UL104 infected cells. The quaternary structure of rpUL104 was investigated by gel permeation chromatography and electron microscopy. The purified rpUL104 was found to assemble into high molecular weight complexes, a prerequisite of portal proteins which form channels for DNA import into capsids.