Abstract 1. 1. Peripheral blood reticulocytes of normal and bled rabbits and of rabbits with phenylhydrazine-induced anemia, were morphologically analysed, through silver sections, for a relative evaluation of hemoglobin (Hb) biosynthesis activity. 2. 2. Reticulocytes of maturation degrees within the range of 35–60 polysomes/μm2, were compared as to their mean numbers of hemosomes (sites of heme integration into the globin chains), and mitochondria (indirect precursors for hemosome formation). 3. 3. The results on the mean numbers of hemosomes per reticulocyte section, correlated to several physiological data under those three conditions, suggested a close relationship between Hb biosynthesis activity and hemosome frequency. 4. 4. In bled rabbits, reticulocytes showing a low mean number of hemosomes ( X hB section = 0.32 ), as compared to reticulocytes of normal rabbits ( X hN section = 0.70 ) and to reticulocytes of rabbits with hemolytic anemia ( X hH section = 2.10 ), gave rise to a new erythrocyte population characterized by a low Hb content. 5. 5. Hb concentration differences were verified by confronting hematological data before bleeding with those obtained after the regression of anemia.