1,2-benzopyrone has been shown to affect on the activation and stimulation of macrophage. To examine the immunostimulating effect of 1,2-benzopyrone on the phagocytic response of canine peripheral blood mononuclear cells (PBMC) as well as polymorphonuclear cells (PMN), the phagocytic activity of phagocytes was analyzed by flow cytometry system using FITC-labelled latex. The 1,2-benzopyrone did not show any direct effect on phagocytic response of PBMC and PMN. But it showed an enhanced effect on the phagocytic response of monocyte-rich cells fractioned by cell size from dot plot profile in flowcytometric cytography of PBMC. The phagocytic activity of these cells was also enhanced by addition of culture supernatant from PBMC treated with 1,2-benzopyrone. Similarly, the phagocytic activity of PMN but not PBMC in the same procedures was enhanced by culture supernatant from PBMC treated with 1,2-benzopyrone. However, the culture supernatant from PMN treated with 1.2-benzopyrone did not show the enhancing effect on phagocytic activity for monocyte-rich cells and PMN. These results, therefore, suggested that enhanced phagocytic activity of canine peripheral blood PMN and monocytes may be mainly mediated by humoral factor(S) released from PBMC treated with 1,2-benzopyrone.