Administration of poly-dispersed acid-functionalized single-walled carbon nanotubes (AF-SWCNTs, 10 µg intravenously on alternate days) induced a sustained anemia in mice. Using a modified double in vivo biotinylation (DIB) technique, blood counts of reticulocytes and erythrocytes of different age groups were simultaneously enumerated in control and AF-SWCNT treated mice. A sustained reticulocytosis was observed in AF-SWCNT treated mice. Young erythrocytes (up to 7 days old) that are normally resistant to elimination in control mice were eliminated at a significant rate in AF-SWCNT treated mice. Old erythrocytes, however, accumulated in circulation indicating that younger erythrocytes were selectively eliminated from the blood circulation of AF-SWCNT treated mice. Cells representing various stages of erythroid differentiation in bone marrow and spleen were enumerated flow cytometrically by double staining with anti-Ter119 and anti-transferrin receptor (CD71) monoclonal antibodies. Proportion of erythroid cells was significantly reduced (up to 27%) in bone marrow (BM) indicating a fall in erythropoietic activity. A concomitant increase in the spleen erythroid population was however observed that could be a compensatory response. Changes in erythroid populations in bone marrow and spleen correlated with changes in erythroblast-A population in these organs that represent an early stage of erythroid differentiation. Uptake of intravenously administered fluorescence tagged AF-SWCNTs (FAF-SWCNTs) was relatively low (3-4%) in erythroid cells in bone marrow and spleen. A significantly higher proportion of pro-erythroblasts and erythroblast-A (early stages of erythroid differentiation) took-up FAF-SWCNTs. Uptake of AF-SWCNTs by early precursors of erythroid differentiation with toxic consequences may be a contributing factor in AF-SWCNT induced anemia.