Simulated vitreous hemorrhages were produced in rabbits by injection of homologous blood tagged with radioactive chromium. The course of the hemorrhage was followed by measuring the rate of change of radioactivity of the eye and by visual and histologic methods. Methods and Materials Rabbit blood was collected in oxalated tubes. The erythrocytes were separated by centrifugation and thoroughly washed with isotonic saline. Radioactive sodium chromate (Na2Cr51O4) was used to label the hemoglobin of the cells by the procedure of Gray and Sterling1 as modified by Reilly et al.2 The tagged erythrocytes were then either suspended in the original plasma to reconstitute whole blood or were laked in a volume of distilled water equal to the initial plasma volume. Hemorrhage was simulated by injection, through a 30-gauge needle, of 0.10 ml. of this blood into the vitreous. Since γ-rays and soft x-rays from the radioactive chromium in