Hemostasis is a complex process that relies on the sensitive balance between the formation and breakdown of the thrombus, a three-dimensional polymer network of the fibrous protein fibrin. Neither the details of the fibrinogen-fibrin transition, nor the exact mechanisms of fibrin degradation are fully understood at the molecular level. In the present work we investigated the nanoscale-changes in the viscoelasticity of the 3D-fibrin network during fibrinogenesis and streptokinase (STK)-induced fibrinolysis by using a novel application of force spectroscopy, named nano-thrombelastography. In this method the changes in the bending of an oscillating atomic-force-microscope (AFM) cantilever in human blood-plasma droplet were followed as a function of time. Whereas the global features of the time-dependent change in cantilever deflection corresponded well to a macroscopic thrombelastogram, the underlying force spectra revealed large, sample-dependent oscillations in the range of 3-50 nN and allowed the separation of elastic and viscous components of fibrin behavior. Upon STK treatment the nano-thrombelastogram signal decayed gradually. The decay was driven by a decrease in thrombus elasticity, whereas thrombus viscosity decayed with a time delay. In scanning AFM images mature fibrin appeared as 17-nm-high and 12-196-nm-wide filaments. STK-treatment resulted in the decrease of filament height and the appearance of a surface roughness with 23.7 nm discrete steps that corresponds well to the length of a fibrinogen monomer. Thus, the initial decay of thrombus elasticity during fibrinolysis may be caused by the axial rupture of fibrin fibers. (C) 2014 Elsevier Inc. All rights reserved.