The aim of this study was to investigate the effects of oligochitosan(OCS),water soluble chitosan (WSC) and acid soluble chitosan(ASC) on the respiratory burst activity of head-kidney phagocytes in Wuchang bream (Megalobrama amblycephala).The head-kidney phagocytes of Wuchang bream were separated by 51% Percoll gradient and the morphological and ultrastructural features were observed under light and electron microscope.Staphylococcus aureus killed by formalin (F-SA) were incubated with phagocytes and observed.PP and PI were counted with Wright Giemsa staining under oil-immersion microscopy.The respiratory burst activity of phagocytes was measured by flow cytometry using dihydrorhodasmine 123 (DHR123) as fluorescence probe after stimulation with phorbol 12-myristate 13-acetate (PMA).The results indicated that the separated cells showed the morphological and functional characteristics of phagocytes.OCS (100 μg/mL),WSC (100 μg/mL) and ASC (100 μg/mL) were all able to stimulate the respiratory burst activity of head-kidney phagocytes after incubation for 30 min.The percentage of fluoresce cell and the fluorescence intensity of fluoresce cell (two parameters of respiratory burst activity) in the experimental group were significantly different (P＜0.01) from the control group.However,the percentage of fluoresce cell of the phagocytes stimulated by WSC was the highest.But the strongest fluorescence intensity was observed in the phagocytes stimulated by ASC.