Zonal centrifugation. The steps are as follows: (A) form a density gradient, (B) layer the sample on top of the gradient, (C) place the tube in a swinging-bucket rotor and centrifuge it, and (D) collect the samples.

[Information from D. Freifelder, Physical Biochemistry, 2d ed. (W. H. Freeman and Company, 1982), p. 397.]