Oligonucleotides are promising therapeutic agents for the prevention or treatment of a variety of diseases. The therapeutic potential of oligonucleotide therapy depends greatly on the bioavailability of oligonucleotides to their target cells and organs. We previously reported the pharmacokinetics and distribution of phosphorothioate oligonucleotide in mice using [35S]-labeled oligonucleotide ([35S]-oligo). To extend this study, we administered 30 mg/kg of fluorescent-labeled oligonucleotide (FITC-oligo) to mice and examined oligonucleotide distribution by measuring the fluorescence intensity in various cells and tissues using flow cytometry. Following FITC-oligo administration, fluorescence was detected in all the tissues examined. In terms of the fluorescent intensity, accumulation was greatest in liver and kidney, intermediate in spleen and bone marrow, and very low in peripheral blood mononuclear cells (PBMC). At 4 hours after administration, the level of oligonucleotide uptake in PBMC, spleen lymphocytes, and bone marrow cells revealed the following pattern: monocytes/macrophages > B cells > T cells. Confocal microscopy detected intracellular fluorescence in PBMC prepared under the same conditions as those for flow cytometry. These studies provide a rationale for designing cell targets for antisense therapeutics.