Antibodies mediating post-attachment virus neutralisation (PN), inhibition of human cytomegalovirus (HCMV)-induced cell fusion in the glioblastoma cell line U373 (IF) and global neutralising activity (NA) were quantified in sera from healthy immunocompetent individuals, asymptomatic HIV-1-infected subjects and AIDS patients to further characterise the neutralising antibody response to HCMV in these population groups and to assess whether HIV-1-infected individuals exhibited an abnormal functional antibody profile. PN and IF antibodies accounted for a minor fraction of the NA activity of sera from all population groups. Sera from HIV-1-infected individuals (particularly AIDS patients) displayed higher levels of PN and IF antibodies than those from the healthy control group; however, the relative contribution of these antibodies to the global serum NA activity appeared to be lower in the former individuals than in immunocompetent controls. Serum antibodies preventing HCMV cell-to-cell spread (IP) were then measured to determine whether a specific deficiency could be detected in the HIV-1-infected group population. Serum IP antibody titres were significantly higher in HIV-1-infected individuals (particularly in AIDS patients) than in controls. The potential implications of the data for explaining the pathogenesis of HCMV infection are discussed.