A systematic, general approach for the design of an initial purification procedure for any biological particle is described in this communication. A series of centrifugations in fixed angle rotors has been used to obtain information on the sedimentation behavior of particles of interest (Anderson, N.G. (1967) Anal. Biochem. 23, 72-83). Refinements of this technique have facilitated the determination of sedimentation profiles of subcellular organelle markers in suspensions of murine spleen and brain. The degree of homogeneity of several particles with respect to size can be ascertained from the sedimentation profiles. Alterations in these profiles after mechanical disruption and treatment with detergents are readily measurable and have been found to be useful in both the characterization and isolation of subcellular particles. Because fixed angle rotors are used in these studies, the data obtained can be directly applied to the development of a preparatory scheme for purification of a desired particle. These methods for sedimentation analysis are readily applicable to subcellular organelles, macromolecular complexes, viruses, viral-like agents, and a variety of macromolecules.