Fibrin accelerates the activation of plasminogen catalyzed by tissue- type plasminogen activator much stronger than fibrinogen. Detailed studies showed that (part of) this rate-enhancing effect of fibrin is brought about by two sites in the fibrin molecule: one in A alpha-(148- 160) and one in the gamma-chain stretch 311–379 (also known as FCB-5). During the fibrinogen-to-fibrin conversion, A alpha-(148–160) appears to become accessible, because a monoclonal antibody against synthetic A alpha-(148–160) reacts with fibrin, but not with fibrinogen. Because a similar situation may exist for (at least parts of) FCB-5, we have prepared a monoclonal antibody against a part (ie, gamma-[312–324]) of FCB-5, and found that this is fibrin-specific and does not bind fibrinogen. We conclude that gamma-(312–324) is hidden in fibrinogen and is exposed by the formation of fibrin.