Fluorescein isothiocyanate (FITC)-conjugated phosphodiester and phosphorothioate oligonucleotides were used in four-color flow cytometry with murine bone marrow cells stained with monoclonal antibody specific for the differentiation markers B220, S7 (CD43), and BP-1 to show possible stage-specific oligonucleotide uptake. Relatively low uptake was observed among pre-Pro- and early Pro-B cells. Late Pro-B- and pre-B cells had increased oligonucleotide uptake, whereas B cells had a lower level. Cell membrane binding of oligonucleotides varied during B-cell differentiation in parallel with internalization, which was documented by confocal microscopy. An FITC-conjugated polyanionic dextran sulfate also showed differentiation-related B-cell association, suggesting the presence of cell membrane binding sites specific for polyanions as opposed to a unique feature of the DNA backbone. Interpretation of antisense experiments in murine bone marrow cells will need to account for the heterogeneous oligonucleotide uptake among differentiating B cells.