Interleukin-2(IL-2)is an important immune regulator.Recently it is found that IL-2 is also an analgesic molecule.Using the potassium iontophoresis as pain stimulus to induce tailflick and taking the intensity of current (mA) at the moment of the response as the pain threshold (PT),it is reported that mutant 20 Leu-IL-2 (20Asp→Leu),which could not bind to the βsubunit of IL-2 receptor and thus had no immune activity, could increase the PT of the rats, indicating that the analgesic and the immunal functions be related to two different domains in IL-2 molecule. Mutant 45Val-IL-2 (45Tyr→Val)，which had immunal activity, had no analgesic effet, suggesting that the 45Tyr be crucial for the analgesic effect of IL-2 and the analgesic domain be located around the 45Tyr.