The present invention is directed to a method of isolating and concentrating biological materials based on their buoyant density by subjecting a sample containing the biological materials to density-gradient ultracentrifugation. In one aspect of the present invention, a method for isolating at least one biological material is provided. A sample containing at least one biological material is introduced into an ultracentrifuge having a density-gradient established therein, and the sample is centrifuged until at least one biological material is isolated according to its buoyant density.