We report the serum levels of soluble interleukin 2 receptor (sIL2R), beta 2-microglobulin (beta 2-M) and interferon-gamma (IFN-gamma) in patients undergoing adoptive immunotherapy with rIL2 and lymphocyte-activated killer (LAK) cells. Our results indicate that rIL2 induced a marked increase of the serum concentration of these markers, although this increase varied considerably for different individuals. Parallel studies with the same patients also showed a marked rise in the number of IL2R+ lymphocytes: the IL2Rs expressed on these cells were mainly of the "low affinity" type. We suggest that evaluation of these markers may allow the monitoring of immune system activation induced by rIL2 in patients undergoing adoptive rIL2 and LAK cell immunotherapy.