Protein transduction domains (PTDs) have been used increasingly to deliver biologically active agents to a variety of cell types in vitro and in vivo. To define the most effective PTDs for transducing hematopoietic cells, we have screened a panel of PTD peptides in human CD34+ cells for delivery of a 60-kd marker protein and assessed its impact on phenotypic maintanence in vitro. Compared to the HIV-TAT peptide, most peptide complexes displayed high efficiency in transducing the CD34+ cells, except for those based on shorter peptides (4R, 4K, and 5RQ). In particular, the arginine homopolymers including 8R, 10R, and 12R, were internalized by the cells to a greater extent than the other PTDs. Transduction was significantly potentiated by preincubation of cells with dextran sulfate. Importantly, colony forming ability and CD34+CD38- primitive phenotype were not significantly altered in the presence of these peptides during a short-term liquid culture. Together, these data suggest the potential usefulness of arginine homopolymers in hematopoietic stem and progenitor cell manipulations.