Dom I and Dom II mabs function during a postadorption step of the infection gB has been postulated to be involved in receptor binding of HCMV and entry [26, 49, 50] . We therefore analyzed at which stage of the infection the mabs exerted their action. To assay influence on attachment, virus/antibody mixtures were added to target cells at 4uC and the number of HCMV DNA copies attached to the cells was determined by quantitative real time PCR. Neither Dom I-nor Dom II-specific antibodies inhibited attachment of virions, indicating that the mabs did not block receptor binding of HCMV (Fig. 5A ). In accordance with previous reports, heparin almost completely prevented virus attachment, whereas the AD-2-specific antibody C23 had no effect on virus binding [27, 51] . The slight increase of bound virus in the presence of some antibodies as compared to control might reflect deposition of antibody/virus aggregates on the surface of cells. We also determined activity of the mabs towards virus that is already adsorbed to cells. To this end, virus was preadsorbed to cells for 1 h at 4uC before antibody was added. Both Dom I-and Dom IIspecific antibodies were capable of neutralizing HCMV at a postadsorption step, whereas non-neutralizing antibodies had no effect (Fig. 5B) . The higher antibody concentration that was required to completely neutralize adsorbed virus is explained by the need to block fusion of an already adsorbed virus particle.