BACKGROUND: Because of a high risk of graft-versus-host disease (GVHD), donor lymphocyte infusions with unmodified lymphapheresis products are not used after haploidentical hematopoietic stem cell transplantation. Natural killer (NK) cells have antitumor activity and may consolidate engraftment without inducing GVHD. Production of NK cells under good manufacturing practice (GMP) conditions in a sufficient number is difficult. STUDY DESIGN AND METHODS: Twenty-four apheresis procedures and subsequent NK-cell enrichment from 14 haploidentical donors were performed. NK-cell enrichment was performed using a GMP suitable immunomagnetic procedure. Factors influencing the NK-cell recovery, purity, and NK-cell dose were analyzed. RESULTS: A median number of 4.9 x 10(8) NK cells were obtained and median NK-cell recovery was 58 percent. Median T-cell depletion was 4.32 log. The absolute NK-cell number in the final product after processing significantly correlated with the preharvest NK-cell content of the peripheral blood (p = 0.002, r = 0.867). The NK-cell recovery was inversely correlated to the absolute NK-cell number in the apheresis product (p = 0.01, r = -0.51). The NK-cell dose per kg of body weight of the patient was inversely correlated to the weight of the patient (p = 0.007, r = -0.533). CONCLUSION: Donors with a high NK-cell count in peripheral blood are likely to provide NK-cell products with the highest cell number. However, maximal NK-cell dose is limited and high NK-cell doses may only be obtained for patients with a low body weight, making children and young adults the best candidates for NK-cell therapy.