Human cytomegalovirus (HCMV) purified from cell culture contains two dominant structural phosphoproteins with apparent molecular weights of 65,000 and 150,000, designated as pp65 and pp150 respectively. The humoral immune response of infected individuals against pp65 is relatively weak and is not always detectable by Western blot analyses. This report shows that recent clinical isolates of HCMV do not necessarily have pp65 as a prominent constituent, suggesting that the low immune reaction is due to variable expression of the pp65 in natural infections. However, the HCMV strains tested in this study produced the large structural phosphoprotein (pp150) in about equal amounts. The pp150 is remarkably immunogenic, if compared with all other virion constituents; serum pools and individual sera from HCMV-infected patients recognized this particular protein intensively in immunoblot assays. Thus, phosphoprotein pp150 seems to be the primary polypeptide candidate for expression cloning in order to develop reagents for novel ways of HCMV diagnosis.