The influence of human cytomegalovirus (HCMV) strain variation on neutralizing antibody titers was investigated in sequential sera obtained from 12 organ transplant recipients (11 renal transplant recipients, 1 liver transplant patient) suffering from primary or secondary HCMV infection. Cross-neutralization assays using either the international HCMV reference strain AD169 or individual clinical isolates from patients showed congruent results. Restriction enzyme analysis of the hypervariable alpha-sequence of the L-S junction of AD169 and HCMV isolates amplified by polymerase chain reaction (PCR) were carried out to confirm the expected strain differences. At least 6 groups of clinical strains were differentiated. The results of this study demonstrated that despite the strain heterogenicity of HCMV, the neutralization assay using AD169 permitted a reliable and quantitative serologic detection of neutralizing antibodies against HCMV.