For the purpose of revealing whether AMD inhibits the RNA synthesis of erythroblasts in an effective dose in vivo to eradicate erythroid cells in rabbit bone marrow, the author observed the RNA synthesis by H3-uridine incorporation in vitro and RNA level on the cells from the anemic animals taken at a certain period after a single injection of AMD in a small dose of 50 and 100μg/kg body weight. The data revealed that by such a small dose of injection of AMD the RNA synthesis of erythroid precursors, early basophilic and proerythroblast stages, was successfully suppressed without any suppressing effect on the RNA synthesis of erythroblasts in the later stages of specialization, indicating that there are at least two kinds of RNA synthesis: one seen mainly in the earlier stages of specialization and the other one seen mainly in the later stages, and they can be distinguished from each other by the AMD sensitivity.