Leukaemia B cell populations, each with an individual pattern of monoclonal surface immunoglobulin expression, were obtained from 23 patients with chronic lymphocytic leukaemia (CLL) and, following exposure to a potent dose of Epstein-Barr (EB) virus in vitro, were monitored for expression of the virus associated nuclear antigen EBNA, for activation of immunoglobulin synthesis and for virus-induced transformation to an established cell line. Although possessing the EB virus receptor, CLL cells were generally refractory (vis-à-vis normal adult B cells) to the full effects of the viral infection. All the leukaemic populations tested developed a small proportion of EBNA positive cells within a few days post-infection, but in most instances this disappeared with no subsequent evidence of viral activity. In certain cases, however, the EBNA staining became more intense, involving a larger fraction of the population and persisting for some weeks, but again this was not accompanied by virus-induced immunoglobulin synthesis or transformation. In contrast, the leukaemic cells from a single patient, tested on three separate occasions, regularly responded to EB virus infection with the rapid establishment of an EBNA positive B cell line in which the restricted pattern of surface and cytoplasmic immunoglobulin expression (gamma lambda) exactly matched that present on the original leukaemic cells.