Noninvasive cellular import of synthetic peptides can be accomplished by incorporating a hydrophobic, membrane-permeable sequence (MPS). Herein, we describe a facile method that expedites synthesis of biologically active, cell-permeable peptides by site-specific ligation of two free peptide modules: one bearing a functional sequence and the second bearing a MPS. A nonpeptide thiazolidino linkage between the two modules is produced by ligation of the COOH-terminal aldehyde on the MPS and the NH2-terminal 1, 2-amino thiol moiety on the functional sequence. This thiazolidine ligation approach is performed with stoichiometric amounts of fully unprotected MPS and functional peptide in an aqueous buffered solution, eliminating the need for additional chemical manipulation and purification prior to use in bioassays. Two different MPSs were interchangeably combined with two different functional sequences to generate two sets of hybrid peptides. One set of hybrid peptides, carrying the cytoplasmic cell adhesion regulatory domain of the human integrin beta3, inhibited adhesion of human erythroleukemia cells to fibrinogen-coated surfaces. A second set of hybrid peptides, carrying the nuclear localization sequence of the transcription factor NF-kappaB, inhibited nuclear import of transcription factors NF-kappaB, activator protein 1, and nuclear factor of activated T cells in agonist-stimulated Jurkat T lymphocytes. In each assay, these nonamide bond hybrids were found to be functionally comparable to peptides prepared by the conventional method. Cumulatively, this new ligation approach provides an easy and rapid method for engineering of functional, cell-permeable peptides and demonstrates the potential for synthesis of cell-permeable peptide libraries designed to block intracellular protein-protein interactions.