SummaryThereisincreasing evidence that Chlamydia pneumoniae is linked to atherosclerosis and thrombosis. In this regard, we have recentlyshown that C.pneumoniae stimulates plateletaggregation and secretion, which mayplayanimportantrole in theprogress of atherosclerosisa nd in thrombotic vascular occlusion. The aims of thep resent study were to investigate thee ffects of C. pneumoniae on platelet-mediated formation of reactive oxygen species(ROS) and oxidationoflow-density lipoprotein (LDL) in vitro.R OS production was registereda sc hanges in 2´,7`-dichlorofluorescin-fluorescence in platelets with flowc ytometry. LDL-oxidationw as determinedb ym easuring thiobarbituric acid reactive substances (TBARs). We found that C.pneumoniae stimulated plateletproduction of ROS.Polymyxin Btreatment of C. pneumoniae,b ut not elevatedt emperature,a bolishedt he stimulatory effects on plateletROS-production, which suggests that chlamydial lipopolysaccharideh as an importantr ole.I n- KeywordsAtherosclerosis, bacteria-cell interaction,o xygen radical,L PS, thrombosis hibitionofnitric oxidesynthase with nitro-L-arginine,lipoxygenasew ith 5,8,11-eicosatriynoic acid and protein kinaseCwith GF 109203X significantlylowered the production of radicals. In contrast, inhibition of NADPH-oxidase with di-phenyleneiodonium(DPI) didnot affectthe C. pneumoniae induced ROS-production.Thesefindingssuggestthat the activities of nitric oxide synthase andlipoxygenase arethe sources forROS and that the generation is dependent of theactivity of protein kinaseC.The C.pneumoniae-induced ROS-production in platelets was associatedwith an extensiveoxidationofLDL,which wassignificantly highercompared to the effect obtainedbyseparate exposureof LDLto C. pneumoniae or platelets.Inconclusion, C. pneumoniae interactionw ith platelets leading to aggregation,ROS-production and oxidativedamage on LDL, mayplayacrucial role in the developmentofatherosclerotic cardiovascular disease.