During the past few years, new approaches to the delivery of functional peptides to cells have been developed to probe intracellular protein-protein interactions. These approaches include a method based on the cell membrane permeability properties of the hydrophobic region of the signal sequence. This method provides easy and rapid delivery of functional peptides to a wide spectrum of cells involved in inflammatory and immune reactions (monocytes, endothelial cells, and T lymphocytes) as well as to NIH 3T3 cells and erythroleukemia HEL cells. The method has been applied to block signaling to the nucleus by transcription factors unclear factor-kappa B, AP-1, and nuclear factor of activated T cells, and to inhibit cell adhesion regulated by the cytoplasmic tails of integrins beta 3 and beta 1. New methods of peptide delivery provide direct access to intracellular proteins involved in adhesion, signaling, and trafficking to the nucleus.