A system is described which allows us to collect flow cytometry sorted objects onto a very small area, thus obtaining very concentrated particles. For that purpose, collection is made directly on a nitrocellulose filter while a vacuum is established below the filter. By comparison with collection onto a microscope slide, the local concentration is increased about 1,000-fold. This efficient and simple system allows us to concentrate cell or cellular organelle populations for observation by microscopy of many objects in a single field, or to study the biological or biochemical properties of a small number of sorted particles.