Physics of Fluids / Volume 24 / Issue 11 / LETTERS / 111702 (2012); (6 pages) Pipe flow experiments show that turbulent drag reduction in plug-flow of concentrated suspensions of macroscopic fibers is a self-similar function of the wall shear stress over the fiber network yield stress. We model the experimental observations, by assuming a central fiber network plug, whose radius is determined by the yield stress. According to the model the plug constrains the size of the turbulent eddies in the surrounding annulus, with the effect of a reduced friction factor as compared to Newtonian flow.