In this study, in vitro and in vivo antitumor effects of mononuclear cells from human umbilical cord blood cells (UCBCs) and peripheral blood stem cells (PBCs) harvest obtained by leukapheresis were compared. Interleukin 2 (IL-2)-activated mononuclear cells from UCBCs showed increased cytotoxicity against K562 and Raji hematopoietic malignant cells compared with PBCs (P < 0.05). After IL-2 activation, both UCBCs and PBCs showed significant cytotoxicity against MDA-231 human breast cancer cells. The UCBC population involved in this antitumor activity appeared to be CD56+ natural killer precursors. The cytotoxicity of UCBCs was inhibited in the absence of Ca2+ (P < 0.05), supporting a perforin/granzyme-mediated target of cell lysis. In addition, antibodies to Fas ligand blocked cytotoxic activity, suggesting that some of the antitumor cytotoxicity was Fas ligand mediated. In vivo antitumor effects of UCBCs and PBCs were studied using a human leukemic cell-bearing severe combined immunodeficient mouse model. There was a significant increase in the survival of K562 leukemia-bearing mice that also received 5 million in vitro IL-2-activated UCBCs or PBCs i.v. on days 3 and day 5 after tumor transplantation compared with untreated mice (P < 0.01). Similar antitumor cytotoxicity of UCBCs and PBCs was also observed against MDA-231 human breast cancer grown in severe combined immunodeficient mice (P < 0.01). These studies suggest that IL-2-activated UCBCs may be a useful source of cellular therapy for patients with hematological malignancies and breast cancer.