We study the dynamics of sedimentation and reentrainment of particles from the umbrella cloud above an axisymmetric, turbulent, particle-laden buoyant plume. We develop a model to show that the reentrainment of particulate material into the uprising plume will cause the particle flux to increase by a factor of e between the plume source and the umbrella cloud. A buoyant plume rising in an environment of uniform density may thereby become negatively buoyant if its particle loading becomes sufficiently high. We compare the predictions of the model to a series of laboratory experiments and show that at high particle loadings the plume undergoes an oscillatory collapse. This periodic collapse generates dense gravity flows down the flanks of the plume. In the context of volcanic eruptions, such an instability, associated with particle recycling may lead to the formation of interleaved fall and flow deposits, such as have been observed near the collapse horizon in a number of pyroclastic deposits. ¿ 2000 American Geophysical Union