5-cis Carbaprostacyclin is a stable analog of PGI2 and an isomer of carbaprostacyclin. It is a weak inhibitor of human platelet aggregation with an IC50 of 2.8 µM compared to 0.3 µM for carbaprostacyclin. 5-cis Carbaprostacyclin is a much weaker effector of rabbit mesenteric artery relaxation with an EC50 of 104 µM compared to 5.9 µM for carbaprostacyclin. It even antagonizes the adenylate cyclase activation induced by carbaprostacyclin.