AbstractPurpose: Epstein-Barr virus immediate early genes BZLF1 and BRLF1 encode the Z and R proteins respectively. Elevation of the anti-Z immunoglobulin (Ig) antibody titer is a common feature of nasopharyngeal carcinoma (NPC) in areas where the disease endemic, whereas R levels have not been examined. This study was performed to analyze antibody titers against Z and R in Japan. Methods: Sera from 14 patients with newly diagnosed NPC, 7 with NPC in complete remission, 15 with head and neck squamous cell carcinoma (HNSCC), and 31 controls were evaluated by enzyme-linked immunosorbent assay (ELISA) and immunoblotting. Results: Both anti-Z and anti-R Ig were significantly increased in sera from patients with NPC compared with those in remission, those with HNSCC, and controls (Z, P=0.0016,=0.0344,=0.0002; R, P=0.0015,=0.0004, 0.0001 respectively). By immunoblot analysis, anti-Z and anti-R Ig were detected in 9 of 9 cases of NPC, 1 of 3 cases NPC in remission, and 2 of 13 cases of HNSCC. Anti-Z and anti-R antibody titers were nine times higher in NPC than in other groups. None of 15 control cases showed positive reactivity against either Z or R. Conclusion: Coupled evaluation of anti-Z and anti-R Ig by ELISA might be a useful system for screening NPC.