Paired Epstein-Barr (EB) virus-carrying cell lines have been established from Burkitt's lymphoma (BL) patients; one the BL-cell line derived from the malignant cells of the tumour and bearing the relevant chromosomal translocation, the other the diploid lymphoblastoid cell line (LCL) derived from the patient's normal B cells by experimental infection with the virus. Comparative studies have shown that both BL and LCL cells are relatively resistant to fresh NK cells but are more susceptible to in vitro-activated NK cells; individual pairs differ as to whether the BL or LCL cells are more sensitive to such effectors. With most cell pairs studied, the two cell types were equally efficient at inducing NK-cell activation in vitro. When the in vitro stimulation protocol was changed to favour the induction of allo-specific responses, most BL cells were noticeably poorer stimulators than the corresponding LCL although both cell types induced a similar pattern of cytotoxicity; this appeared to be directed against HLA class I allo-antigens and could be inhibited by class I antigen-specific monoclonal antibodies. Most BL target lines, though susceptible to this allo-specific lysis, were significantly less so than the corresponding LCL However, only 1/7 BL-cell lines tested vis-à-vis the LCL gave evidence of a gross reduction in HLA antigen expression and this was immediately apparent from both serological typing and cellular analysis.