Antibodies to Epstein-Barr virus (EBV)-capsid antigen (VCA), early antigen (EA) and EBV-associated nuclear antigen (EBNA) in the sera of 103 patients with adult T-cell leukemia (ATL) and the sera of 243 age- and sex-matched healthy adults, namely 99 anti-ATLA-positive (antibodies to ATL virus-associated antigen) and 144 anti-ATLA-negative individuals, were determined by indirect immunofluorescence. The anti-VCA titers in the sera from ATL patients were within the range observed in healthy controls. Anti-EA was found in 27% of the sera from ATL patients, but in only 8% of the sera from anti-ATLA-negative healthy controls. Antibodies to EBNA, which were present in almost all healthy adults, were not found in 30% of the sera from ATL patients. Of the sera of anti-ATLA-positive healthy adult donors 11% were negative for EBNA antibody. These results suggest that functional impairment of the T-cell system in most ATL patients, and also to a lesser extent in anti-ATLA-positive adults who might be healthy ATLV-carriers, may cause an unusual immune response of antibodies to EBV-associated antigens.