Magnetic Separation of Phagosomes of Defined Age from Tetrahymena thermophila

. We describe a new mass isolation procedure for both pure and stage‐specific phagosomes from Tetrahymena thermophila . We prepared magnetic iron dextran particles about 1 μm in diameter to label the phagosomes. The oral apparatus of the cells concentrated these particles so readily that after 1 min the majority of the cells had formed a single phagosome. A short wash removed non‐ingested particles, enabling us to follow the age‐dependent changes of a single labeled phagosome through the cell. Phagosomes of different ages, including very young and nascent phagosomes, were removed easily from the non‐magnetic cell debris of mechanically homogenized cells by means of a permanent magnet. The isolated phagosomes are pure as tested by enzymatic assays and light and electron microscopy. Since the yield of pure phagosomes of all ages is high (∼ 90%), this method could be generally applied for phagosome isolation from ciliates.