Sera from 24 patients with nasopharyngeal carcinoma (NPC) and from 60 normal subjects were examined for the presence of IgG and IgA antibody to various antigens associated with Epstein-Barr virus (EBV). The results indicate that NPC in Cuba, like in other countries, was accompanied by high titers of EBV-specific IgG antibody the levels of which differed markedly from those in normal subjects. Serological activity of the patients differed depending on the stage of the illness and its duration. Geometric mean antibody titers to the capsid antigen (VCA), D and R components of early antigen (EA) of EBV complex were higher in patients with extensive tumour process (III--IV stages) and in patients with the duration of the disease since the onset less than 3 years. Sera of the most patients contained EBV-specific IgA antibody which were practically completely absent in sera of the control subjects. Alongside with characteristic manifestations of responsiveness to EBV observed by other workers, patients with NPC in Cuba had specific features: higher antibody titers to VCA than in Caucasian subjects in the countries where this disease occurred sporadically, like in Cuba; high levels of antibody to EA, frequently exceeding those in patients with NPC from endemic zones (for instance in Chinese patients); prevalence of EBV-specific antibody to the R component of EA complex.