Abstract: The capacity of peritoneal exudate cells (PEC) obtained from mice infected or immunized with Plasmodium berghei to produoe reactive oxygen species and its related biological basis was investigated, using luminol-dependent zymosan-triggered chemiluminescence (CL). CL response of PEC from infected mice increased at the early stage but was significantly dcpressed at the later course of the infection. A similar biphasic activity of peroxidase was observed in PEC from infected mice. On the other nand, PEC from immunized mice exhibited concomitant increases of the ability to produce CL, the activity of peroxidase and the expression of Fc and C3 receptors on cell surface. Compared with the controls, PEC from immunized mice showed an elevated CL background, responded more rapidly to the stimulation and generated considerably higher CL when triggered with opsonized zymosan. The data suggest that phagocytes in immunized mice are aotive in the production of reactive oxygen species while those in the infected ones are less active, and the inhibition of the ability of phagocytes to produce reactive oxygen species may be one of the me-chanisms for the parasites to escape the host immune system.