SynopsisZone centrifugation experiments are occasionally ruined by convective processes which occur a t one gravity subsequent to layering a solution of macromolecules on a supporting density gradient. The factors which contribute to this convection have been little understood. In most of the experiments described in this communication, Blue Dextran 2000 is used as the macromolecular component. The convective disturbance may be clearly seen and accurately measured. The effects of macromolecular concentration, composition and density of the supporting gradient, diffusion coefficient of the solute, and several other variables are studied, and an empirical equation constructed to describe the phenomenon. T h i s convective disturbance appears to be minimized or prevented by layering and fractionating in the presence of a centrifugal field.When a solution of large molecules is layered at one gravity over a more dense solution containing a low molecular weight solute, small streaming columns of the macromolecular solution may form at the interface and sink through the dense solution below. An understanding of the conditions governing this phenomenon is important for zone centrifugation studies.In this communication, we will examine some of the factors upon which the convective disturbance depends. In order to do this, a measurement of some reproducible quantity associated with the streaming phenonemon is required. It has proven to be convenient to measure the time interval which occurs after layering, but before the formation of streaming columns of macromolecules. This time lag may be determined with some precision. By showing how the time lag varies with concentration, diffusion coefficient, temperature, etc., we may determine those parameters of the system which are critically involved in the initiation of the streaming phenomenon.