Phagocytosis of eel neutrophils isolated from peripheral blood was measured by chemiluminescence (CL) assay. When neutrophils were exposed to opsonized bacteria, CL response increased rapidly. CL response to the non-opsonized bacteria was much lower than that to the opsonized ones. The amount of CL response became larger in proportion to the concentration of normal eel sera as opsonin. The CL activity varied with the individual fish; neutrophils from three fish appeared to be similar in CL response, but those from a fish were weaker in the response. In this study, it was found that the CL assay could be applied not only to investigate the magnitude and kinetics of phagocyte but also to assess the serum opsonic activity.