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The effects of hydrogen peroxide（H

2

O

2

）on endothelial-polymorphonuclear leuko-cyte（EC-PMN）adhesion and their mechanisms were studied in cultured bovine pulmonaryartery endothelial monolayers in vitro.H

2

O

2

at various concentrations(10

-3

10

-2

10

-1

mol/Lrespectively)stimulated EC-dependent PMN adhesion

of which 10

-2

mol/L H

2

O

2

was the mostpotent one

increasing adhesion to 2.3 times that of the control.Pretreatment of PMNs with SRI63-441

a platelet-activating factor（PAF）receptor antagonist

had no inhibition effect on H

2

O

2

induced EC-PMN adhesion.Pretreatment of ECs with SRI 63-441 before H

2

O

2

exposure signifi-cantly decreased PMN adherence to ECs.Pretreatment of ECs with phospholipase A

2

inhibitorp-bromophenacyl-bromide or calmodulin antagonist chlorpromazine and calcium ion chelate EG-TA obviously decreased H

2

O

2

induced increment of EC-PMN adhesion.These results suggestthat H

2

O

2

may activate ECs

causing the inflow of extracellular calcium or the release of calciumfrom intracellular deposits.Increased intracellar Ca

2+

may bind with calmodulin to activate phos-pholipase A

2

thus initiating PAF synthesis and promoting EC-PMN adhesion.