Circulating immune complexes (CIC) isolated from sera of mice infected with Plasmodium berghei by precipitation with polyethylene glycol were examined for their ability to stimulate the production of reactive oxygen species by peritoneal exudate (PEC) of normal mice, using luminol-aided chemiluminescence (CL). CIC were found to be capable of stimulating the production of CL by normal mouse PEC. Weaker CL responses were observed when PEC were incubated with P. berghei soluble antigens. Normal mouse IgG and IgG1, IgG2b subclasses showed no effect on the stimulation of the production of CL. When normal mouse PEC were preincubated with CIC, their CL responses to opsonized zymosan were significantly depressed to 34% of the control level. The data suggest that CIC are capable of stimulating phagocytes to release reactive oxygen species and mediating pathological tissue damage.