Histidine-rich glycoprotein (HRG) is a serum protein belonging to the cystatin superfamily. HRG may play a regulatory role in hemostasis and innate immunity. However, this role is uncertain because of a lack of rigorous testing in an animal model. We generated mice lacking the translation start point of exon 1 of the Hrg gene, effectively resulting in a null mutation (Hrg-/-). The mice were viable and fertile but had no HRG in their blood. Antithrombin activity in the plasma of Hrg-/- mice was higher than in the plasma of heterozygous Hrg+/- or wild-type Hrg+/+ mice. The prothrombin time was shorter in Hrg-/- mice than in Hrg+/- and Hrg+/+ mice. Bleeding time after tail tip amputation in Hrg-/- mice was shorter than in Hrg+/+ mice. The spontaneous fibrinolytic activity in clotted blood of Hrg-/- mice was higher than in Hrg+/+ mice. These findings suggest that HRG plays a role as both an anticoagulant and an antifibrinolytic modifier, and may regulate platelet function in vivo.