Aptamers have emerged as a new class of small molecule ligands. These short, single-stranded oligonucleotides can be produced through simple chemical synthesis, making them easier and less costly to produce than antibodies. We synthesized an RNA aptamer probe specific for human CD4 using a reported sequence and investigated the potential use of this probe in cell phenotyping. Studies in cultured cells demonstrated that the synthetic CD4 aptamer had a nearly identical cell-binding specificity as the standard CD4 antibody. Fluorescent microscopy confirmed that the aptamer and antibody generated the same CD4 staining pattern in cells without competing with one another. Multicolored flow cytometty analysis revealed that the CD4 aptainer could be combined with antibodies to phenotype cells from bone marrow, lymph nodes, and pleural fluid, suggesting that the aptamer probe has value for clinical use.