Fibrinogen mediates endothelial cell adhesion, spreading, and angiogenesis through integrin α_vβ₃. Previous studies by several investigators have suggested that the Arg-Gly-Asp (RGD) site at position 572−574 on the α chain of human fibrinogen can bind to α_vβ₃. However, this RGD sequence is absent in fibrinogen from most other species, including bovine, hamster, monkey, mouse, pig, and rat fibrinogen. In these species, an RGD site exists at the equivalent of position α252−254, which has the sequence RGG in humans. In addition, the role of an integrin binding site on the γ chain at position 400−411 has been an issue of controversy. In the present studies, recombinant fibrinogen molecules with mutations in the potential endothelial cell binding sites have been used to test the role of these sites directly. The results show that the RGD at α572−574 is the primary adhesion site, and that the γ chain site plays no significant role. Human and bovine plasma fibrinogens were also assayed for their ability to support adhesion of human and bovine vascular endothelial cells. The results show that although the two types of fibrinogen have RGD sequences at widely divergent sites, there is no significant difference in their ability to support endothelial cell adhesion. Furthermore, a chimeric human fibrinogen molecule with an RGD sequence at the bovine site, position α252−254, also supported adhesion. These results indicate that an RGD site in human fibrinogen at either position α252−254 or position α572−574 can mediate endothelial cell adhesion.