It is well known that hyperthermia (HY), which is used for the treatment of cancer, depresses natural cell-mediated immunity in vitro. Experiments were performed to confirm the inhibitory effect of HY (42 degrees C for 1 hour) on natural killer (NK) activity and to evaluate the influence of HY on the generation and cytotoxic activity of interleukin-2 (IL-2)-activated NK cells. Additional experiments were also carried out to evaluate the effect of a simultaneous exposure of effector and target cells to HY. The results showed that HY profoundly reduced the lytic activity of NK cells and demonstrated that this inhibition was transient and not due to an apoptosis-induced reduction of the number of effector cells. Moreover, the exposure of mononuclear cells to HY before IL-2 stimulation did not affect the generation of IL-2-activated NK cells, whereas, the hyperthermic treatment of IL-2-activated NK cells produced a marked reduction of their cytotoxic activity. The results also showed that the simultaneous exposure of effector and target cells to HY, during the cytotoxicity assay, produced a marked reduction of lytic activity of NK and IL-2-activated NK cells, and that this impairment was specific for effector cells. In this context, heat-exposure of target cells alone, did not substantially modify their susceptibility to lysis induced by either NK or IL-2-activated NK cells. These results add further evidence of HY-induced inhibition of natural cell-mediated immunity, and suggest that, in the course of therapeutic HY, immune response could be significantly altered.