The CD4+CD25high regulatory T (Treg) cells have been demonstrated to negatively modulate anti-tumor immune responses in cancer patients. In this study, effects of low dose anti-CD25 antibody (Ab) to attenuate Treg cells were investigated in cancer patients in vitro and in vivo. Peripheral blood mononuclear cells (PBMCs) from cancer patients were cultivated in vitro in the presence of a high-affinity chimeric anti-CD25 Ab (basiliximab). The CD4+CD25high population, interferon-gamma (IFN-gamma) production and FOXP3 expression were analyzed using flow cytometry (FCM), enzyme-linked immunosorbent assay and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis, respectively. During in vivo studies, basiliximab was administered intravenously on day 1, followed by AIT using autologous activated lymphocytes on day 8, and the treatment cycle was repeated. Subjective and objective effects were observed, and patients' PBMCs were subjected to FCM and RT-PCR analysis. In vitro analysis revealed that a low concentration of 0.01 microg/ml basiliximab reduced almost all of CD4+CD25high cells, but less of the CD4+ CD25low cells, and augmented IFN-gamma production of activated PBMCs. FOXP3 mRNA expression of PBMCs was not affected with or without basiliximab. An in vivo study of 9 metastatic cancer patients (7 colorectal and 2 esophageal) demonstrated no subjective or objective adverse effects, even under repeated administration of basiliximab. The results suggested that low-dose basiliximab can safely be administered repeatedly, and can target CD4+CD25high Treg cells whilst relatively preserving CD4+CD25low activated T cells. The host conditioning with low-dose basiliximab may augment the efficacy of AIT for cancer using activated autologous lymphocytes.