THE method described in this note provides a quick and efficient means for the fractionation of highly disperse systems—including the larger proteins—using relatively moderate centrifugal forces. Furthermore, all necessary information about the sedimentation rate and the sedimentation equilibria of these systems can be obtained, samples for examination being taken before and after spinning. Many important biological agents (viruses, bacteriophages, antibodies, etc.) remained outside the scope of the admirable methods developed by Svedberg because they are not susceptible to direct continuous optical observation, but they are within the range of the new procedure.