Activated natural killer (A-NK) cells, a subset of CD56(dim)CD3- lymphocytes, are obtained from PBMC of normal donors by adherence to plastic and culture in the presence of IL2. In this study we tested the feasibility of generating A-NK cells in patients with Ph+ chronic myeloid leukaemia (CML). Cultures obtained from patients with early chronic phase (ECP; n=7) contained a mean (+/-SD) of83 +/- 7% of CD3- cells, and those from patients with advanced chronic phase (ACP; n=7) contained 27+/-33% CD56+CD3- cells. In three patients with leukaemia in a blastic phase (BP) it was only possible to obtain one culture enriched in CD56+CD3- cells (81%). Cellular aggregates of myeloid cells and large granular lymphocytes were observed in early A-NK cell cultures. Paired freshly-adherent and cultured A-NK cells were tested for the presence of BCR/abl mRNA by RT-PCR. The BCR/abl+ cells were detected in all 12 preparations of the freshly adherent A-NK cells tested. In 6/12 the BCR/abl+ cells were no longer detectable by RT-PCR on day 14 of culture. Both proliferation and antileukaemic cytotoxicity were significantly higher (P=0.002 and P=0.029, respectively) in the BCR/abl- cultures than those in the six BCR/abl+ cultures. 5/6 BCR/abl- cultures were highly enriched in A-NK cells on day 14, and 1/6 contained predominantly CD56+CD3+ cells. Only 2/6 BCR/abl + cultures were enriched in A-NK cells on day 14, but they had poor cytotoxicity and a low proliferative index. Myeloid cells (CD33+) were more frequently detected in the BCR/abl+ than BCR/abl- A-NK cell cultures (P=0.028). These observations suggest that: (1) populations of benign A-NK cells can be generated from the peripheral blood of CML patients; (2) the ability to generate A-NK cells is impaired in patients with advanced CML; and (3) the ability to generate A-NK cells with antileukaemic activity correlates with the disappearance of BCR/abl+ cells from these cultures.