Glycoprotein B (gB) of human cytomegalovirus (HCMV) was partially purified by lentil-lectin column chromatography from cells infected with an adenovirus-gB recombinant. This antigen, which contained specifically reactive proteins of approximately 130 and 55 kDa, was used to investigate gB antibody levels after natural HCMV infection in 48 individuals. All sera had IgG antibody to gB as detected by radioimmunoprecipitation (RIP) assays. Quantitative RIP showed a strong correlation between gB antibody and neutralizing activity (r = .74, P less than .001) but a weak correlation between gB antibody and total HCMV-specific IgG (r = .36, P less than .02). When gB antibody was specifically absorbed from 20 serum specimens, neutralizing antibody titer was reduced a median of 48% (range, 0-98%). These data confirmed that antibodies to gB are a large component of the neutralizing antibody response to HCMV and support a role for this protein in the development of subunit vaccines.