The ultrastructural changes which occurred during red cell degradation in the vitreous were studied in rabbit and human eyes. The major pathway for red cell lysis was extracellular in both species, and this type of haemolysis was probably induced by the unfavourable environment in the vitreous and by age changes in the red cells themselves. The released red cell debris was then phagocytosed by invading macrophages. Intracellular lysis also occurred within the cytoplasm of large macrophages after ingestion of whole red cells by the phagocytes. Thus inflammatory cells were always ultimately involved in both forms of red cell lysis in the vitreous. Intracellular lysis appeared to be more common in human cases of recurrent intravitreal haemorrhage associated with intravitreal new vessel formation. In addition, a higher proportion of young macrophages was observed in the inflammatory cell exudates of recurrent haemorrhages, suggesting that macrophage turnover was more rapid in these cases.