An enzyme-linked immunosorbent assay was used to quantify soluble interleukin 2 receptor (IL2R) in the serum of 25 patients prior to allogeneic or autologous bone marrow transplantation and in the early post-transplantation period. A significant correlation between IL2R and the occurrence of fever and/or graft-versus-host disease (GVHD) could be shown. Patients with moderate to severe GVHD (grade II-IV) had significantly higher IL2R levels (median 480 U/ml) than patients without or with acute GVHD grade I (median 139 U/ml). In patients without or with acute GVHD grade I, significant differences in the maximum IL2R levels depended on the duration of fever greater than or equal to 38 degrees C. Evaluation of the peak IL2R levels in patients with fever lasting longer than 6 days led to a median of 260 U/ml and in patients with fever lasting less than 6 days to a median of 118 U/ml. In patients without or with acute GVHD grade I, who developed fever lasting longer than 6 days, IL2R levels started to rise with the onset of fever, reached peak values during temperature maximum and declined parallel with temperature normalization. In patients without or with acute GVHD grade I who developed fever lasting for only 6 days or less IL2R levels remained within the normal range. In patients with acute GVHD grade II-IV, IL2R levels began to rise with the onset of fever, and then continued to rise despite temperature normalization. The peak levels were reached in the early period of acute GVHD. Our observations in BMT patients show that severe infections and acute GVHD are associated with a stimulation of the immune system leading to elevated IL2R serum levels.