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Ellagitannins (esters composed of glucose and ellagic acid) are hydrolyzed to generate ellagic acid in gut followed by conversion of ellagic acid to urolithins such as urolithin A by intestinal bacteria. Since urolithins are absorbed by gut easier than ellagitannins and ellagic acid, and show various physiological activities (e.g. anti-cancer, anti-cardiovascular disease, anti-diabetes mellitus, anti-obesity and anti-Alzheimer disease activities), they are expected as excellent health-promoting phytochemicals. Here, using human monoblast U937 cells, we investigated the effect of ellagic acid and urolithin A on the superoxide anion (O2−)-generating system of phagocytes, which is consisted of five specific protein factors (membrane proteins: p22-phox and gp91-phox, cytosolic proteins: p40-phox, p47-phox and p67-phox). Twenty micromolar of urolithin A enhanced the all-transretinoic acid (ATRA)-induced O2−-generating activity (to ~175%) while 20 μM ellagic acid inhibited the ATRA-induced O2−-generating activity (to ~70%). Semiquantitative RT-PCR showed that transcription level of gp91-phox was certainly decreased (to ~70%) in ATRA plus ellagic acid-treated cells, while that of gp91-phox was significantly increased (to ~160%) in ATRA plus urolithin A-treated cells. Chromatin immunoprecipitation assay suggested that urolithin A enhanced acetylations of Lys-9 residues of histone H3 within chromatin surrounding the promoter region of gp91-phox gene, but ellagic acid suppressed the acetylations. Immunoblotting also revealed that ATRA plus urolithin A-treatment up-regulated protein levels of p22-phox (to ~160%) and gp91-phox (to ~170%) although ATRA plus ellagic acid-treatment down-regulated protein levels of p22-phox (to ~70%) and gp91-phox (to ~60%). These results suggested that conversion of ellagic acid to urolithin A in gut may bring about reverse effects on the gp91-phox gene expression, resulting in opposite alterations in O2−-generating activity of intestinal macrophages.