An apparatus and method for the separation of a predetermined type of microscopic unit (e.g., islets of Langerhans) from a given medium (e.g., pancreatic digest) by centrifuging in a substantially continuous density gradient. In one embodiment, a substantially continuous density gradient solution is provided to a flexible processing bag appropriately positioned within an upper chamber of a rotating variable volume centrifuge. The solution having a plurality of the desired microscopic units therein is loaded onto the gradient and such units migrate to a particular radial position within the gradient based upon the density of such units. Once appropriately separated by the formation of regions, a fluid is provided to a lower chamber of the centrifuge bowl during continued rotation thereof to sequentially remove fractions from the processing bag from an inward location to an outward location, at least one of which fractions includes the band of the desired microscopic units. The desired microscopic units of a given fraction(s) may therefore be provided to an appropriate collector.