A biologic sample such as feces, sputum, cervical tissue, pleural fluids, exudates, cytologic specimens, or the like, is tested for the presence or absence of: ova; parasites; microorganisms; inflammatory, neoplastic tissue cells; or other target materials which are indicative of infestation, disease or infection. The sample is mixed with a buffer fluid and placed in a transparent tube which contains a volume-constricting cylindrical insert for gravimetric separation of components of the sample. The mixture is centrifuged, and the annular space between the insert and tube bore is examined under magnification for the presence of the target materials.