Effect of SI level and saturation of TIBC on the induction of ferritin in erythroid cells was investigated in rats, guinea pigs, and rabbits. Evidence presented indicates that species difference and serum iron level are important factors in induction of micropinocytotic ferritin in developing erythroid cells: (1) Micropinocytotic ferritin was occasionally seen in erythroblasts of control guinea pigs. Erythroblasts of control rats and rabbits were negative. (2) Elevated SI and high saturation of TIBC increased micropinocytotic ferritin in developing erythroid cells in guinea pigs. Iron administration to rats and rabbits did not affect the accumulation of micropinocytotic ferritin. In guinea pigs, disappearance of stromal ferritin during erythroid maturation was demonstrated. High SI level did not affect the stromal ferritin of erythroid cells. Dissociation of ferritin increase in micropinocytotic vesicles of erythroblasts on the one hand and in macrophage cytoplasm on the other was seen when iron-sorbitol was injected. The results emphasize the biologically dissimilar nature and kinetics of morphologically different types of ferritin in erythroblasts.