The effects of endotoxin on protein phosphorylation, calcium transportationand fibrinogen receptor exposure in human platelets were studied. The proteins from ~(32)P-labled human platelets were separated by SDS-polyacrylamide gel electroporesis, then theprotein phosphorylation were observed with autoradiographic method. The result showedthat endotoxin rapidly induced phosphorylation of 20kD protein. An enhanced phosphory-lation of 20kD protein was seen in the presence of extracellular Ca~(2+). Meanwhile, endo-toxin induced an increase of the cytoplasmic free Ca~(2+) concentration and promoted ~(45)Ca_(2+)uptake by platelets. In addition, the platelets to which FITC-labled fibrinogen was boundhad increased. It suggested that the mechanism of endotoxin induced platelet aggregationinvolved in Ca~(2+) mobilization and protein phosphorylation, which caused the fibrinogenreceptor exposure.