The tegument of human cytomegalovirus (HCMV) contains a phosphorylated protein of 65 kilodaltons, termed pp65, which was reported to carry significant epitopes for the stimulation of the humoral immune response during natural infection. A monoclonal antibody directed against this protein was used to screen a lambda gt11 cDNA library for recombinant polypeptides. Two DNA fragments from purified lambda clones and one fragment from genomic DNA were used for cloning in a bacterial high-level expression vector. The resulting fusion proteins were tested for their reactivity with a panel of monoclonal antibodies directed against pp65 and with polyspecific anti-HCMV rabbit antisera. The binding site for all the monoclonal antibodies tested was found to be contained in one of the recombinant proteins with a viral portion of 26 amino acids. Immunoblot analyses with HCMV-positive human sera revealed that pp65 alone is not a reliable antigen for serodiagnosis but may be very useful in combination with other HCMV proteins.