To measure the intracellular ferritin (Ft) content in human erythroid cells, a new method was developed using human bone marrow amears. With this method, Ft, transferrin receptors (TfR) and stainable iron granules (SIG) were semiquantified at the single cell level and the cell's maturation level was identified. The relationship between Ft and other parameters was investigatted. Ft-H subunit concentrations were found to decrease with maturation from basophilic to orthochromatic erythroblasts (r=-0.62,p<0.05) and further decreased after denucleation. There was significant positive correlation between the Ft-H subunit concentra-tion and TfR density (r=0.52,p<0.01), but the relation between the Ft-L subunit and TfR or the density of SIG was obscure. These data indicate that : 1) The Ft-H subunit might play a more important role than the Ft-L subunit during iron metabolism and differentiation of erythroid cells. 2) Some factors other than intracellular iron levl, such as maturation, seem to play important roles in the Ft expression in erythroid cells.