AIM: To investigate the effect of lipopolysaccharide (LPS) priming on macrophage(M?). METHODS: Macrophage cell line RAW264.7 were pretreated with or without LPS for 1 h, then challenged with PMA, or LPS, muramyl dipeptide(MDP), Zymosan, formyl-methionyl-leucyl-phenylalanine(FMLP) for 1 h . O  2 production in supernatants and intracellular free calcium([Ca 2+ ]i ) were measured, and changes in [Ca 2+ ]i and LPS induced O  2 production were compared. RESULTS: LPS pretreatment significantly increased O  2 production in RAW264.7 cells challenged with the stimuli, and in a certain extent, both O  2 production and increase of resting intracellular [Ca 2+ ]i were dose- and time-dependent on LPS pretreatment.Furthermore,the peak [Ca 2+ ]i was significantly higher in LPS pretreated groups than that of LPS unpretreated groups when challenged with PMA. Pretreatment with Ca 2+ inophore A23187 mimicked the LPS priming effects on O  2 production, but pretreatment with Ca 2+ chelator BAPTA and EGTA blocked this priming effect. CONCLUSION: LPS induced M? priming effect on O  2 production is dependent on elevation of resting intracellular [Ca 2+ ]i .