Two related but differentially expressed potential membrane proteins of Epstein-Barr virus are encoded by the same reading frame in the EcoRI D het region of the viral genome. Potential antigenic sites in the amino acid sequence of these proteins were selected by computer-aided prediction of the secondary structure and two oligopeptides corresponding to regions located in different parts of the proteins were synthesized chemically. Rabbit antisera to these peptides were used for immunoprecipitation of the native viral proteins from Epstein-Barr virus-positive cell lines from various sources. Both predicted membrane proteins could be precipitated from cell lines that had been transformed in vitro with EBV or from cell lines derived from infectious mononucleosis patients. In cell lines established from Burkitt lymphoma, only the smaller polypeptide, which lacks 138 amino acids from the amino terminus, could be identified. Using the synthetic peptides as antigens in ELISA, we detected elevated antibody titers in sera from patients with infectious mononucleosis and nasopharyngeal carcinoma.