Fibrin, but not fibrinogen, enhances the rate of activation of plasminogen by tissue type plasminogen activator (t-PA). Studies with enzymatic and chemical fragments of fibrinogen showed that several sites in fibrinogen are involved in this rate enhancement; these are, A alpha 148-160 (located in CNBr fragment FCB-2), and FCB-5 (a CNBr fragment comprising gamma 312-324), and recently discovered sites in the fibrinogen alpha C domains. All these sites are buried in fibrinogen, but exposed in fibrin and some fibrinogen fragments. For the first two of these, located in the D-domains, this was shown by the fact that monoclonal antibodies against A alpha 148-160 and gamma 312-324 bind to fibrin and rate enhancing fibrin(ogen) fragments, but not to fibrinogen. Direct binding studies indicate that at physiological concentrations plasminogen binds to FCB-2, and t-PA to FCB-5. More detailed studies have demonstrated the importance of residues A alpha-157 and A alpha-152, and that the minimum stretch with rate enhancing properties is A alpha 154-159. The sites in the alpha C domains await further identification. With the recently reported three-dimensional structure of fragments D and D-dimer it is now possible to explain these findings at the molecular level. Molecular calculations and experimental data show that the site A alpha 148-160 in fibrinogen is covered among others by a part of the A alpha chain (A alpha 166-195) that forms an alpha-helix, and by a globular domain formed by the beta-chain. On fibrin formation, the last two may move away, and give access to A alpha 148-160. It is conceivable that in the alpha C domain sites are involved in the early phases of fibrinolysis. The site A alpha 148-160 and that in FCB-5 may be more important at later stages. It is also clear that fibrin polymerization is important. This polymerization has probably several effects: exposure of the rate enhancing sites; mutual positioning of the t-PA and plasminogen binding sites; a concentrating effect of t-PA and plasminogen on the fibrin surface; effects on the kinetic properties of t-PA and plasminogen. These effects together explain the rate enhancement.