The aim of this study is to determine the phagocytosis-promoting factor(s) for feline peripheral blood polymorphonuclear cells (PMN) by culture supernatant from mono-nuclear cells (MNC) treated with egg white derivatives (EWD). The phagocytic activity of PMN was analyzed by a flow cytometry system. The EWD did not show direct effect on the phagocytic response of PMN. The phagocytic activity of PMN was enhanced by culture supernatant from MNC but not PMN treated with EWD. Therefore, it was suggested that the enhanced phagocytic activity of feline PMN could be mediated by humoral factor(s) released from MNC treated with EWD. Thus, the phagocytosis-promoting factor(s) in supernatant fraction from MNC culture treated with EWD were isolated by reverse phase high pressure liquid chromatography. The resulting supernatant fraction on 29.02 minutes of retention time showed high phagocytic activity of PMN. The molecular weight of this supernatant fraction was 16 to 18 kDa when analyzed by capillary electrophoresis. The isoelectric point was pH 5.76 when assessed by ion-exchange chromatography. These results suggest that EWD stimulates feline MNC to elaborate a phagocytosis-promoting factor, 16 to 18 kDa of molecular weight, which could be an important mediator for the enhancement of phagocytic activity of feline peripheral blood phagocytes. Further study will be needed to elucidate this phagocytic factor,