Stimulated by a report on elevated IgA levels in nasopharyngeal carcinoma (NPC), we tested a total of 372 sera from patients with NPC, other carcinomas of head and neck or elsewhere, Burkitt's lymphoma (BL), infectious mononucleosis (IM) or healthy controls. The sera were titrated in indirect immunofluorescence tests for IgA antibodies to Epstein-Barr virus (EBV) capsid antigen (VCA) and to the diffuse (D) or restricted (R) components of the EBV-induced early antigen (EA) complex. The results proved NPC to be outstanding in that prior to therapy 93% of the patients tested revealed IgA antibodies to VCA and 73% to D, often at high titers which occasionally matched the corresponding IgG antibody levels. The EBV-specific IgA titers increased from stages I or II to stages III or IV; i.e. with the total tumor burden. Conversely, many of the NPC patients examined 2–6 years after initial therapy had only low levels of EBV-specific IgA or none at all, and the majority of those with high titers were known to have residual or recurrent disease. In contrast to untreated NPC patients, less than 5% of 73 patients with other carcinomas or of 76 healthy donors revealed VCA-specific IgA and even fewer EA-specific IgA; only 28% and 4% of 54 BL patients tested at admission had IgA antibodies to VCA and R, respectively, and 38% and 3% of 37 IM patients showed transient VCA-or D-specific IgA responses, all at generally low titers. While sera from untreated NPC patients often contained IgA antibodies also to herpes simplex type 1 virus, their incidence and range of low titers were similar to those obtained with sera from patients with other carcinomas or from healthy donors. It thus appears that the elevated IgA levels in NPC might be due to EBV-specific antibodies. Possible reasons for this unique response in NPC have been discussed.