The immunostimulating effects of egg white derivatives (EWD) on the phagocytic response of feline peripheral blood polymorphonuclear cells (PMN) as well as mono- nuclear cells (MNC) were examined. The phagocytic activity was analyzed by a flow cytometry system. The EWD showed directly an enhanced effect on the phagocytic response of MNC but not PMN. The phagocytic activity of MNC was enhanced by culture supernatant from MNC and PMN treated with EWD, respectively. Similarly, the phagocytic activity of PMN was enhanced by culture supernatant from MNC but not PMN treated with EWD. It was, therefore, indicated that the enhanced phagocytic activity of feline PMN could be mainly mediated by humoral factor(s) released from MNC treated with EWD. These results suggested that EWD could enhance the phagocytosis of feline peripheral blood phagocytes.