A device for the direct measurement of cohesion, which is the shear strength of a powder when no normal stress is applied to the plane of shear, is discussed. The apparatus, called the Warren Spring-Bradford cohesion tester, works according to the principle of the annular shear cell. The central part is a hollow cylinder to which eight spokes are attached, connected by an outer ring. Two materials, lactose and soda ash, have been tested extensively in a wide range of sizes. By plotting the cohesion versus the mean size, a clear distinction can be made between free-flowing powders, characterized by a size-independent cohesion, and cohesive powders that exhibit a cohesion the value of which is a strong function of particle size. The size at which the transition between the two states occurred was approximately 55 mu m. Apart from dense copper powder the values of the cohesion could be correlated satisfactorily with those derived from the Jenike shear cell. This cohesion tester served its intended purpose very well, i.e. obtaining a qualitative comparison as regards the flowability of powders of different sizes.