A systematic approach for selecting centrifuges and rotors is explained on the basis of their performance parameters. While a parametric numberδST, essentially a function of sedimentation coefficient and rotor speed, is used to characterize batch centrifugation, an analogous parametric numberSTYis introduced to specify continuous centrifugation and modified to describe semicontinuous centrifugation. A graphic technique is explained for determining the maximum possible output from a semicontinuous flow centrifuge; this procedure permits convenient selection of the optimal core for a given rotor bowl.