We have compared the proliferative and cytotoxic capacities of a highly purified population of recombinant interleukin-2 (rIL-2)-activated peripheral blood mononuclear cells (PBMNC), termed adherent lymphokine-activated killer cells (A-LAK), in 15 chronic phase (CP) and 10 advanced disease (AD) Ph-positive chronic myelogenous leukemia (CML) patients. The selective enrichment of CML A-LAK cells depended on their propensity to adhere to plastic and to proliferate when cultured in the presence of rIL-2 for 14 days. In both CP and AD patients, 14-day culture resulted in growth of a uniform population of large granular lymphocytes. While less than 10% of the A-LAK cells were CD56-/CD3+ (mature T lymphocytes), 82% ± 12% of A-LAK cells from early CP patients (diagnosed less than 1 year from study), 84% ± 3% of A-LAK cells from late CP patients (studied greater than 1 year after diagnosis), and 87% ± 3% of A-LAK cells from AD patients were CD56+/CD3- (activated natural killer [NK] cells). No bcr gene rearrangement could be found in A-LAK cells from 13 CP and six AD CML patients studied. A-LAK cells from seven early CP CML patients displayed similar cytotoxicity against K562 (80% ± 7% lysis at effector:target ratio of 20:1) and against Raji (80% ± 12% lysis) compared with A-LAK from 17 normal individuals (72% ± 3% K562 lysis, P = .21; 74% ± 5% Raji lysis, P = .39). However, the cytotoxicity of A-LAK cells from eight late CP patients (59% ± 5% K562 lysis, P = .02; 52% ± 8% Raji lysis, P = .02) and that of 10 AD patients studied at any point after diagnosis (31% ± 3% K562 lysis, P < .001; 25% ± 6% Raji lysis, P < .001) was significantly lower than that of seven early CP CML patients and 17 normals. The proliferative potential of A-LAK cells from seven early CP CML patients (291 ± 191-fold) was significantly greater than that of A-LAK cells from 17 normal individuals (23 ± 3-fold, P = .03), eight late CP patients (46 ± 17-fold, P = .02), and 10 AD patients (5.4 ± 1.9-fold, P = .01). In contrast to CML A-LAK, K562 cytotoxicity of unstimulated mature peripheral blood NK cells was significantly lower in early CP CML patients than in normals and remained low at all stages of disease. A population of benign, highly cytotoxic, hyperproliferative IL-2 sensitive mononuclear cells termed A-LAK can be generated from the peripheral blood of early CP CML patients. Either prolonged duration of CP disease or occurrence of advanced disease is associated with significant diminution of both cytotoxic and proliferative capacities of the A-LAK population that cannot be explained by malignant transformation of the A-LAK cells. Furthermore, the disparity in early CP CML patients between the markedly diminished K562 cytotoxicity found in the circulating unstimulated, mature NK cells compared with that of A-LAK cells suggests that the A-LAK culture method reveals a population of NK progenitors whose cytotoxic potential is not accurately reflected in conventional cytotoxicity assays of mature peripheral blood NK cells. Exploration of the defects in cytotoxicity of killer cells obtained after A-LAK culture may illuminate mechanisms underlying escape from control associated with disease progression in CML and other leukemias.