Publisher Summary When particles are subjected to zonal sedimentation, they have a broad distribution and hence, are very much diluted. This can be a serious problem in the fractionation of small amounts of tissue. The usual methods of fractionation by differential sedimentation involve not the formation of a pellet at the bottom of a tube initially filled with a particle suspension. Such pellets are contaminated by small particles that were initially near the bottom of the tube. This contamination can be reduced by resuspending the pellet and recentrifuging it. The success of this procedure depends on the extent to which the particles can be resuspended without agglutination of particles and damage to particles during resuspension. Such resuspension is very difficult, if not impossible, when the particle suspension to be fractionated is very dilute as, for example, is usually the case in the fractionation of tissue culture cells. To circumvent the practical difficulties of resuspending very minute pellets and of the great dilution inherent in usual zonal differential sedimentation, one alternative is the use of a viscosity barrier, a thin layer of high-viscosity fluid.