Serum antibodies against Epstein-Barr virus (EBV)-determined antigens have traditionally been titrated by the indirect immunofluorescence (IIF) technique. The avidin-biotin complex (ABC) immunocytochemical technique was used to determine the serum levels of IgA against EBV viral capsid antigen (IgNVCA) and IgA against EBV early antigen (IgAIEA) in sera of 106 nasopharyngeal carcinoma (NPC) patients prior to treatment and 100 normal individuals. The sensitivity of the ABC technique is enhanced by an amplification of the antigen-antibody reaction, which involves the binding of the enzyme-linked ABC to the second biotinylated antibody. There was a good correlation (r = 0.9988) between ABC and IlF-determined IgNVCA-positive titres, with the ABC technique being more sensitive than IlF in the detection of IgNVCA in NPC sera: 94% (99/106) and 76% (80/106), respectively. The frequency of IgA/EA reactivity in NPC sera was a Is0 markedly increased by i mm u nodetection with the ABC technique as compared with IIF technique: 63% (691106) and 28% (301106) respectively. Both the immunocytochemical techniques were equally specific in discriminating between elevated serum titres of IgNVCA and IgNEA in NPC sera from normal human sera.