Epstein-Barr virus-related antibody titres were determined in twenty-seven patients with ataxia-telangiectasia (AT) and twenty-two healthy members of their families, in twenty-two patients with other diseases, among them ten with Behçet's disease and ten with various primary immune deficiencies, and fifteen healthy members of their families, and in twenty-three unrelated healthy controls. The AT patients showed an increased incidence (55.6%) of high antibody titres (greater than or equal to 1:320) to viral capsid antigen (VCA), and also a high incidence (48.2%) of antibodies to Epstein-Barr virus (EBV) induced early antigens (EA), but low titres (less than 1:10) of antibodies to the EBV-associated nuclear antigen (EBNA) in 44.2% of the cases. The geometric means of anti-VCA were three-to four-fold higher, and of anti-EBNA six-fold lower, than those of the control groups. The patients with the other diseases did not differ significantly from the controls except for a higher incidence of anti-EBNA titres of less than 1:10 (38.1% vs 4--5%). AT patients with low anti-EBNA titres tended to have more advanced T cell deficiencies than AT patients with moderate anti-EBNA titres, as detected by counts of total lymphocytes and E-rosetting cells, and skin test responses. The results support the hypothesis that a functioning T cell system is required to release EBNA from EBV genome-carrying cells for initial and maintained production of anti-EBNA.