Antithymocyte globulin (ATG) is commonly used in allogeneic haematopoietic stem cell transplantation (HSCT). Little information is available, however, as to the optimal protocol for use and the side-effects occurring if ATG is administered in high daily doses (10-30 mg/kg). We report our experience with ATG Fresenius (ATG-F) in conditioning for allogeneic HSCT. During a period of 3 days, 47 patients received doses between 10 and 30 mg/kg either over 4 h preceded by 1-1.5 mg/kg prednisolone 30 min before the start of ATG-F (protocol A) or alternatively, over 12 h with 3-4 mg/kg prednisolone being administered before and 6 h after start of ATG (protocol B). During treatment with ATG-F, the side-effects observed included inflammation, disseminated intravascular coagulation, hyperdynamic circulation and renal dysfunction. Although these complications caused substantial morbidity, they were reversible within a few days. Side-effects were significantly more severe in patients treated according to protocol A than in those treated according to protocol B. As prolonged infusion of ATG-F does not reduce T cell clearance due to the long half-life of ATG-F, and since less cytokine release during conditioning might have beneficial long-term effects, we recommend administering ATG-F over 12 h preceded by high-dose steroid treatment.