Paired Epstein-Barr virus (EBV)-carrying cell lines have been established from Burkitt's lymphoma (BL) patients, one of each pair being the BL cell line derived from the malignant cells of the tumour, the other, the lymphoblastoid cell line (LCL) derived from the patient's normal B cells by experimental infection with the virus. Comparative studies have shown the following: (1) All the lines were to some extent sensitive to in-vitro activated natural-killer cells, individual pairs differing as to whether BL or LCL cells were more susceptible. (2) For six of the seven pairs tested, the BL cell line was clearly sensitive to allo-specific (anti-class 1 HLA) effector T cells, although levels of lysis were slightly below those observed for the corresponding LCL; only one BL cell line showed evidence of a dramatic reduction of HLA antigen expression, and this line was insensitive to allo-specific cytolysis. (3) For two of the three pairs tested to date, EBV-specific cytotoxic T-cell preparations from HLA antigen-matched donors lysed the LCL but not the BL cell line, despite the latter's apparent expression of the relevant restricting antigens. In both of these cases, it was known that the tumour arose in vivo in the face of prevailing EBV-specific T-cell surveillance. An escape of the malignant cells from such surveillance may therefore be important in the overall pathogenesis of EBV genome-positive BL.