Fibrin is formed at sites of tissue injury and provides the temporary matrix needed to support the initial endothelial cell responses needed for vessel repair. Basic fibroblast growth factor (bFGF) also acts at sites of injury and stimulates similar vascular cell responses. We have, therefore, investigated whether there are specific interactions between bFGF and fibrinogen and fibrin that could play a role in coordinating these actions. Binding studies were performed using bFGF immobilized on Sepharose beads and soluble 125I-labeled fibrinogen and also using Sepharose-immobilized fibrinogen and soluble 125I-bFGF. Both systems demonstrated specific and saturable binding. Scatchard analysis indicated two classes of binding sites for each with Kd values of 1.3 and 260 nM using immobilized bFGF; and Kd values of 0.9 and 70 nM using immobilized fibrinogen. After conversion of Sepharose-immobilized fibrinogen to fibrin by treatment with thrombin, bFGF also demonstrated specific and saturable binding with two classes of binding sites having Kd values of 0.13 and 83 nM. Fibrin binding was also investigated by clotting a solution of bFGF and fibrinogen, and two classes of binding sites were demonstrated using this system with Kd values of 0.8 and 261 nM. The maximum molar binding ratios of bFGF to fibrinogen were between 2.0 and 4.0 with the four binding systems. We conclude that bFGF binds specifically and saturably to fibrinogen and fibrin with high affinity, and this may have implications regarding the localization of its effect at sites of tissue injury.