In order to assess the differences in serological reactivities of NPC patients from high, intermediate and low incidence areas, 288 NPC sera from Hong Kong, Singapore, Tunis, East Africa, Paris and Los Angeles, together with sera from patients with ear, nose and throat tumours other than NPC and with those from normal individuals from the same areas, were tested 'blind' with the same batches of antigen. Important differences (up to 3-fold) in the GMTs of antibodies directed against VCA, EA and EBNA were observed between different ethnic groups. Although the stages of the disease varied somewhat between groups, the main cause of the variations appeared to lie in the socio-economic environment of the patients. Apart from these variations, and regardless of geographical or ethnic origins, NPC was consistently found to be associated with an active infection (or reactivation) by Epstein-Barr virus, which was not the case for patients with other tumours or for normal individuals from the same areas or of the same ethnic groups.