1. A microspectrophotometric apparatus was used to measure nuclear absorption in fixed and stained human bone marrow smears. 2. The validity of the measurements was established. 3. The total and polymerized desoxyribose nucleic acid (DNA) of erythroblast nuclei were measured in relative units, and a gradual loss of DNA from the red cell nucleus during maturation was noted. 4. The loss in DNA is comparable quantitatively to that occurring during pathological pyknosis in sarcoma cells. 5. The continual loss of nuclear material during erythropoiesis supports the conception that the nucleus disappears by intracellular dissolution, not by extrusion in the intact form.