Several proteins have been accorded the unusual ability to translocate across cell membranes in a receptor-independent and temperature-independent manner, and this activity has been mapped to a highly basic series of residues currently termed a 'protein transduction domain' (PTD). This translocatory attribute, if authentic, would be valuable for purposes of gene therapy and vaccination. We have evaluated the PTD from the human immunodeficiency virus type 1 (HIV) tat protein and we conclude that, when synthesized de novo, (1) the HIV tat PTD does not enhance the immunogenicity of a full-length protein to which it is tethered; and (2) the HIV tat PTD does not cause intercellular transfer of an attached marker protein, as judged by careful quantitative analyses. From our data, and from a review of published materials, we suggest that contrary to current dogma there is little evidence that these supposedly translocatory proteins can move between live cells. Furthermore, we suggest that PTDs do not act to enhance translocation, but instead merely to increase binding to the cell surface; in which case, the term 'protein transduction domain', and the related acronym, are misnomers which should be abandoned. Our conclusions explain why the most dramatic demonstrations of PTD efficacy have been obtained using fixed cells and/or denatured proteins, and have obvious implications for gene therapy and vaccination.