Previous reports have provided evidence for the ability of human cytomegalovirus (HCMV) to bind the host protein beta 2 microglobulin (beta 2m) from body fluids or culture medium, and thus enhance infectivity of the virus, both by evasion of immune neutralization and the capacity to employ the bound beta 2 m for attachment to the host cell. Immunocytochemical techniques and negative stain electron microscopy were used to identify the ultrastructural components of HCMV involved in its interaction with beta 2m. Probes comprising colloidal gold coupled to beta 2m were seen to bind not to the envelope as previously suspected, but to material closely surrounding the nucleocapsids. It is postulated that the tegument proteins of HCMV, via their capacity to bind beta 2m, play an important role in the preservation of infectivity of disrupted virions by enabling unenveloped capsids to bind to cells and gain entry by a pathway other than that normally taken by intact virions.