Summary: In order to study the role of annexin Ⅱ , a recombinant expression vector, pZeoSV2 (+)ANN Ⅱ , containing the annexin Ⅱ cDNA, was developed. The 1.1-kb-length annexin ⅡeDNA was inserted into a expression vector, PZeoSV(+) and transfected into HL-60 cells which had low baseline expression of Ann- I. pZeoSV(+) ANN I was analyzed by restriction mapping and the Ann-I sequence identified. The ability of the transfected cells, non-transfected and mock-transfected cells to stimulate t-PA-depend plasminogen activation was compared. The results showed that HL-60 with pZeoSV (+)ANN I transfection could significantly increase the plasminogen activation (8. 9± 1.2 U) in vitro with the difference being significant as compared with non-transfected (1.5±0. 4 U) and mock-transfected cells (4.2±0. 9 U), respectively. Antiannexin Ⅱ oligonucleotides significantly inhibited the binding ability of t-PA and plasminogen to annexin Ⅱ , and obviously reduced the plasminogen activation in vitro. The above findings showed human umbilical vein endothelial cells (HUVECs) treated with sense or missense oligonucleotides indicated no significant change in binding of t-PA and PLG. Treatment of HUVECs with antian nexin Ⅱ oligonucleotides could significantly reduce the plasminogen activation by 2.4± 0. 3 U as compared with sense oligonucleotide group in binding of t-PA and PLG. These results, therefore,suggest that Ann- Ⅱ can bind plasminogen and participate in the stimulation of t-PA-dependent ac tivation of plasminogen, and that interference with Ann- Ⅱ mRNA by antisense oligonucleotidemay be a new strategy for the therapy of bleeding in patients with hyperfibrinolysis.