The zonal rotor ultracentrifugation technique is superior to swingingbucket rotors for the separation of larger quantities of subcellular particles in continuous-density gradients. Since large gradient volumes are needed and radius dependence is encountered in forming the desired gradient, gradient mixers designed for filling zonal rotors are different from those used for swinging-bucket rotors. Such gradient formers are commercially available (e.g., Beckman model 141; MSE type Z 100; IEC catalog No. 3652 or 3659)) but much too expensive. Various mixers described in the literature (l-5) use motor-driven precision pumps which cause pulsation and, in the case of an obstruction, considerably high pressures. In this paper we describe a gradient-forming apparatus that utilizes compressed inert gas rather than a pump. The advantages of this device are precisely controlled pressure, stepless variation of flow rate, and ready availability of required parts. Calculations for the formation of nearly linear gradients as well as various exponential gradients are given. The estimation of centrifugation times for various gradients and temperatures is demonstrated. Agreement between results obtained and calculated is shown in the case of the separation of Escherichia coli ribosomal subunits.