We investigated the release of superxide anion ( O2- ) in human granulocytes stimulated by leukocyte factor ( LF ) and/or concan-avalin A ( Con A ). Some amount of O2- can be released by granulocytes, when LF or Con A was applied alone. The pretreat-ment of granulocytes with a low concentration LF1 potentiated O2-release in granulocytes stimulated by Con A, and the lag time of O2-release was reduced. The results show that the granulocytes were activated by LF. This phenomenon may be related to the curative effects of LF.