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 less than .001; 25% +/- 6% Raji lysis, P less than .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.