This chapter describes a new strategy for the delivery of full-length proteins and peptides into mammalian cells based on a short amphipathic peptide carrier, Pep-1. This peptide carrier allows the delivery of several distinct proteins and peptides into different cell lines in a fully biologically active form, without the need for prior chemical covalent coupling or denaturation steps. Pep-1/macromolecule particle is favored as soon as it has crossed the cell membrane. This peptide-based protein delivery strategy presents several advantages, including rapid delivery of proteins into cells with very high efficiency, stability in physiological buffers, lack of toxicity, and lack of sensitivity to serum. Detection of fluorescently labeled antibodies in cells requires a large amount of antibody to be delivered. Depending on the sensitivity required to detect antibodies, it may be necessary to increase the amount of Pep-1 and antibody used. The advantages of the Pep-1 technology are directly associated with the mechanism through which this carrier promotes the delivery of proteins and peptides into cells.