An electron microscopic study of initial X-ray changes in ultrastructures of bone marrow cells was performed. Such changes reached nucleous and cytoplasm organellas early (1 hour) after radiation, but were more proeminent later on. Subsequent readings (2, 6 hours) showed them more proeminent in cytoplasm structures, and the morphologic appearance was close to normal within 12, 14 days. Quantitative analyses were performed using the marrow recovery coefficient (MRC).