In order for human cytomegalovirus (HCMV) to replicate, concatemeric DNA has to be cleaved into unit-length genomes and packaged into preformed capsids. For packaging to take place and DNA to be translocated, a channel is required in the capsid. Viral capsid channels are generally formed by portal proteins. Here, we show by cross-linking, native gel electrophoresis of infected cells and gel permeation chromatography that the HCMV portal candidate protein pUL104 can form dimers and higher order multimers. Electron microscopy of purified monomeric pUL104 after 5 min incubation revealed that the protein had assembled into a multimeric form and that this form closely resembles complete portal assembly. This is the first study to show that pUL104 monomers have the ability to form portal complexes without additional viral proteins.