Fordham University 1 article published in JoVE Immunology and Infection Large-Scale Purification of Porcine or Bovine Photoreceptor Outer Segments for Phagocytosis Assays on Retinal Pigment Epithelial Cells Célia Parinot1,2,3, Quentin Rieu1,2,3, Jonathan Chatagnon1,2,3, Silvia C. Finnemann4, Emeline F. Nandrot1,2,3 1INSERM, U968, 2Sorbonne Universités, UPMC Paris 06, UMR_S 968, Institut de la Vision, 3CNRS, UMR_7210, 4Department of Biological Sciences, Center for Cancer, Genetic Diseases and Gene Regulation, Fordham University This article describes the protocol for the purification of photoreceptor outer segment fragments (POS) via ultracentrifugation from porcine/bovine retinae using homogenization and sucrose gradient centrifugation. This protocol allows the preparation of large stocks of POS aliquots, labeled or unlabeled, that can then be stored at -80 °C.