Epstein-Barr virus (EBV) is the aetiological agent of infectious mononucleosis and is associated with Burkitt's lymphoma and nasopharyngeal carcinoma¹. The virus is harboured for life in all previously infected individuals² and is apparently controlled by a population of EBV-specific memory T lymphocytes, specifically activated to recognize the functionally defined lymphocyte-detected membrane antigen³. Two types (A and B) of EBV have been identified that show DNA sequence divergence within the BamHl WYH region of the genome encoding the transformation-associated antigen, Epstein-Barr nuclear antigen 2 (EBNA 2) (ref. 4). To define the function of EBNA 2 in T-cell recognition, we have compared the ability of EBV-specific cytotoxic T-cell clones to distinguish between autologous B lymphocytes transformed by A-or B-type virus. We have now isolated both CD4 and CDS cytotoxic T-cell clones that recognize autologous A-type but not B-type transformed lymphoblastoid cell lines, thus providing the first evidence that EBV-specific T-cell recognition can be mediated by EBNA 2. As this antigen is not expressed in Burkitt's lymphoma⁵, this finding explains the failure of EBV-specific T-cell surveillance to eliminate the tumour.