The neutralization of a potent anti-factor Xa pentasaccharide (SR 80027) and heparin-induced bleeding enhancement by protamine sulphate was studied in vivo. Bleeding time, as measured by transection of the tail of anaesthetized rats, increased after the administration of standard heparin and SR 80027. Doses of 0.6 and 2.5 mg/kg of heparin and SR 80027, respectively, were required to enhance blood loss to the same extent (6-fold increase). Protamine sulphate (10 mg/kg i.v.) reduced blood loss induced by both compounds but also neutralized the anti-factor Xa activity as well as the antithrombotic activity of standard heparin measured in a venous thrombosis model. However, protamine sulphate did not affect the anti-factor Xa activity or the antithrombotic activity of SR 80027. These data suggest that protamine sulphate may be an effective antidote for the bleeding side-effects of SR 80027 but they also indicate that the bleeding tendency associated with this type of compounds cannot be attributed to their anti-factor Xa activity.