Although thymoglobulin and alemtuzumab are frequently used in hematopoietic stem cell transplantation (HSCT), little is known of their effects on NK cells, which mediate important functions in post-transplantation immunology. In the present study, we determined NK cell death in vitro using propidium iodide and Annexin V. The NK cell activity in 34 patients at day +30 after allogeneic HSCT was assessed using the CD107a assay. Alemtuzumab and thymoglobulin were similarly very potent in inducing NK cell death in vitro. Even in low concentrations (<1 microg/ml) the antibodies induced apoptosis and necrosis in a relevant percentage of NK cells (>30%). However, the number of tumor reactive (CD107a+) NK cells was 13.16 per mul and 1.15 per microl (mean) in patients receiving T-cell depletion with 6 mg/kg thymoglobulin and in patients receiving 100 mg alemtuzumab, respectively (P=0.02). Although thymoglobulin and alemtuzumab are equally NK cell toxic in vitro, the recovery of NK cell frequency and anti-tumor reactivity is reduced in recipients of alemtuzumab. Our findings can be explained by a longer half-life of alemtuzumab as compared to active thymoglobulin under therapeutic conditions. Prolonged immunosuppression with increased risk of infections and tumor relapse are a potential threat to patients undergoing HCST and receiving alemtuzumab as T-cell depletion.