Fibrin aggregation is of vital importance in many physiological and pathological processes, such as blood coagulation, wound healing, and thrombosis. In the present study, we investigated the forces involved in the initial steps of the fibrinogen fibrin aggregation by force spectroscopy using the atomic force microscope. Our data confirm the existence of strong specific interactions between fibrin and fibrin (ogen), with unbinding forces ranging from 290 to 375 pN and a logarithmic dependence on the loading rate between 0.8 and 23 nN/s. (c) 2007 Wiley Periodicals, Inc.