Interactions between fibrinogen molecules proteolytically cleaved with thrombin were investigated using atomic force microscopy (AFM) and dynamic light scattering. (DLS). Gradually decreased fibrinogen concentrations were used to study the fibrin network, large separated fibrils, small fibrils in the initial association stages, and protofibrils. In addition, a new type of structure was found in AFM experiments at a low fibrinogen concentration (20 nM): the molecules in these single-stranded associates are arranged in a row, one after the other. The height, diameter, an distance between domains in these single-stranded associates were the same as those in the original fibrinogen molecules. DLS data assumed formation of, extended associates in bulk solution at fibrinogen concentration as low as 20 nM.