Abstract: Head kidney macrophages from large yellow croaker were isolated by density gradient sedimentation and characterized under light microscope and electronic microscope. Cells were maintained in Leibowitz L-15 medium for 5 days at an optimal temperature of 22℃. The typical macrophages, rich in phagosomes and mitochondria, exhibited strong adhesion and plasticity as well as high phagocytic capacity of engulfing yeasts. Macrophages were incubated with lipopolysaccharide (LPS) from Escherichia coli 055:B5 (0, 0.1, 1, 20 μg/mL) for 3h and 24h, respectively. After the exposure, reactive oxygen intermediates (ROI) production was determined. Only 3h incubation with 0.1 μg/mL LPS could significantly enhance ROI production of PMA-triggered macrophages compared to control cells (LPS = 0 μg/mL), but after 24h incubation, ROI production was significantly decreased by all levels of LPS (P 0.05). At the absence of PMA, significant decrease in ROI production was found in the treatment with 1 μg/mL LPS, while no obvious effects were observed in treatments with other levels of LPS.