We generated a homogeneous population of cells with cytotoxic activity termed “adherent lymphokine-activated killer” (ALAK) cells from the peripheral blood of nine patients in the chronic phase of Ph1 positive chronic myelogenous leukemia (CML). The selective enrichment of CML ALAK cells depended on their propensity to adhere to plastic and proliferate when cultured in the presence of recombinant interleukin-2 (rIL-2) for 14 days. Culture of peripheral blood mononuclear cells under these conditions resulted in growth of a uniform population of cells with morphologic characteristics of large granular lymphocytes. The NKH1+/CD3- phenotype associated with IL-2-stimulated natural killer (NK) cells was present on 79% +/- 9% of cells. Absence of colony formation in conditions promoting the growth of CFU-GEMM indicated that the CML ALAK population was not contaminated with viable hematopoietic progenitors. Cytogenetic analysis of the CML ALAK population revealed 119/120 Ph1 negative metaphases and l/120 Ph1 positive metaphase in six patients. Southern blot analysis of CML ALAK failed to demonstrate a bcr gene rearrangement in seven patients known to have a bcr gene rearrangement in myeloid cells. Comparison of ALAK populations derived from peripheral blood of CML patients and normals revealed similar cytotoxicity against the NK-sensitive K562 cell line (104 +/- 36 LU v 88 +/- 19 LU; P = NS) and the NK-resistant Raji cell line (93 +/- 26 LU v 98 +/- 28 LU; P = NS). The proliferative capacity of CML ALAK cells (101 +/- 33 fold expansion) exceeds the growth potential of the normal ALAK cells (22.3 +/- 3 fold expansion; P = .02). Direct comparison of equal numbers of CML ALAK cells and a CML LAK cell population produced by incubation of peripheral blood mononuclear cells in rIL-2 for 14 days without adherence revealed that the CML LAK population had significantly lower lytic activity against K562 and Raji cell lines. We are able to expand CML peripheral blood mononuclear cells to provide a population of ALAK cells with potent cytotoxic activity. The CML ALAK population is relatively homogeneous, not contaminated with viable stem cells, not derived from a malignant lineage, and more cytotoxic than equal numbers of CML LAK cells. Further studies are underway to determine if this ALAK population may be effective in autologous killing of chronic myelogenous leukemia stem cells.