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An isokinetic gradient is a density and viscosity gradient, in which the particles during the centrifugation sediment with constant velocity. A numerical method for construction of isokinetic sucrose gradients for gradient centrifugation in B XIV zonal rotors is described, and several examples of gradients for separation of soluble proteins, nucleic acids, ribosomes and polysomes are given. The general usefulness of isokinetic gradients is illustrated by experimental separations of soluble immune complexes and of rat brain RNA.According to the definition by No11 [l] an isokinetic gradient is a gradient in which the increase in the driving force acting on a particle with increasing distance from the centre of the rotor is exactly compensated by an equivalent increase in the opposing forces of viscous drag and buoyancy. Noll has pointed out the advantages in using these gradients for gradient centrifugations, and he has developed the theoretical background for, and a gradient forming device t o be used in connection with swing-out rotors.Previously, we [2] have proved the value of isokinetic sucrose gradients for separation and analysis of soluble immune complexes by rate-zonal ultracentrifugation in a B X I V zonal rotor. For these experiments, the gradient was constructed by simulating the centrifugations on a computer, using different types of convex sucrose gradients. However, this method does not necessarily give an optimal gradient, and in unfortunate cases the simulations may take quite a long time.I n the present paper a numerical method for the construction of optimal isokinetic sucrose gradients for rate-zonal centrifugation with a zonal-rotor is described. For these calculations a BXIV zonal rotor [3] has been used as a model, and the gradients are meant for production in an automatic, profile reading gradient former. Isokinetic sucrose gradients for separation and analysis of soluble proteins, nucleic acids, and ribosomes and polysomes, respectively, are described. MATERIAL AND METHODSI n the experimental work a B XIV aluminium rotor and a Superspeed 50 TC ultracentrifuge (Measuring and Scientific Equipment Co

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