Experiments have been conducted using spherical glass particles transported in a recirculating now loop with internal pipe diameter of 40.9 mm by water and by ethylene glycol solutions. Particle diameters were 1.8 and 4.6 mm and the solids concentrations ranged from 9% to 38% by volume. Pressure gradients have been interpreted in terms of a particle friction factor which depends upon factors first identified by R.A. Bagnold in experiments conducted by shearing neutrally buoyant suspensions in a concentric cylinder viscometer.