Lymphocytes are notoriously difficult to transfect. The favoured technique, electroporation, has three major disadvantages: it is highly disruptive, causing large scale cell death; it is inefficient; quiescent primary lymphocytes are refractory to electroporation unless they have been partially activated. We have investigated the cellular import of the third helix of the Antennapedia homeodomain protein (pAntp) as an alternative method for introducing peptides into primary lymphocytes and lymphoid cell lines. The pAntp peptide is taken up rapidly into the cytoplasm and nucleus of the cells where it is retained for at least 48 h. The system displays none of the disadvantages of electroporation; no toxicity of the pAntp peptide was detected at the concentrations tested and the process was efficient with up to 95% of lymphocytes importing the pAntp peptide. Finally, quiescent primary lymphocytes were as efficient as activated primary lymphocytes and a lymphoid cell line at importing the pAntp peptide. This demonstrates that the pAntp peptide delivery system has major advantages over electroporation as a method of delivering molecules to lymphocytes and a lymphoid cell line.