It has been suggested that protamine sulfate is a poor antidote for the bleeding side-effeets of low molecular weight heparins (LMWHs) in vivo, since protamine sulfate does not completely neutralize the anti-factor Xa activity of LMWHs in vitro or ex vivo. Therefore, we performed experiments to compare directly the abilities of protamine sulfate to neutralize the anticoagulant activities of the LMWH, enoxaparine, and unfractionated heparin ex vivo, with its ability to neutralize the bleeding side-effeets of both compounds in vivo. Bleeding was measured as the amount of blood lost from 5 cuts made in rabbits ears before and after treatment with enoxaparine or unfractionated heparin ± protamine sulfate. Plasma anti-factor Xa and anti-thrombin activities ex vivo, were measured chromogenically. Doses of 400 and 1,500 anti-factor Xa U/kg of heparin and enoxaparine, respectively, were required to enhance blood loss to the same extent. Protamine sulfate completely neutralized blood loss induced by both compounds, but did not neutralize the anti-factor Xa nor antithrombin activities ex vivo. We conclude that protamine sulfate is an effective antidote for the bleeding side-effeets of enoxaparine and unfractionated heparin, despite its inability to completely neutralize their anticoagulant activities.