Abstract: According to the present invention, there is provided a method for the induction of a NK cell-mediated immune response ex vivo characterized in that a physiological cell suspension containing at least tumor cells or animal or human cells infected by viruses, bacteria and/or fungi (target cells) and NK cells is treated by the following process steps in the order mentioned to increase the sensitivity of the target cells for lysis by NK cells: Heat treating the cells contained in the suspension at a temperature of 38° C. to 43° C. for a period of at least 1 hour; lowering the temperature to physiological cell temperature (about 37° C.) to give the cells contained in the suspension a recovery period of at least 1 hour; addition of a compound increasing the portion of membrane-bound Hsp7O of the target cells in a concentration which is sublethal for the cells and allowing the compound to be effective for at least 30 minutes; recovery period of at least about 1 hour at 37° C.