Bone marrow from a patient with bilateral staphylococcal bronchopneumonia who was treated with large doses of chloramphenicol was examined with the electron microscope. Our study supports the currently accepted concept that this drug depresses protein synthesis in mitochondria as it does in bacteria, since numerous alterations of mitochondrial morphology were encountered. These include (1) swollen mitochondria; (2) mitochondria with ruptured envelope; (3) mitochondria with longitudinally oriented cristae; (4) "ring" mitochondria; and (5) mitochondria with intramatrical iron deposits. The electron microscopical study discloses that the well documented vacuoles seen in marrow cells on light microscopy after treatment with this drug represent lipid droplets.