Intracellular delivery using cell penetrating peptides is a promising approach for the transport of biomacromolecules and particulate carriers across cell membranes into cells to modify cellular functions. Synthetic peptides with novel backbone design were examined for their ability to form complexes with and transport small interfering RNA (siRNA) into the cell cytoplasm. These new compounds are peptidomimetic dodecamers based on alternating repeats of chiral N-alkylated ß-alanine residues (ß-peptoids) and a-amino acids with a net charge of +6. Complexes of siRNA and peptidomimetics were prepared by mixing and characterized with respect to size and surface charge. At ratios of peptide nitrogen to siRNA phosphate (N/P) of 1 and below, particles with narrow size distributions (poly dispersity indexes lower than 0.11) ranging from approximately 100 to 350 nm were formed, and they showed a negative zeta potential (-24 to -31 mV). At higher N/P ratios, larger aggregates with zeta potential close to neutral were formed. However, the complexes were not able to silence the expression of enhanced green fluorescent protein (EGFP) in HeLa-cells stably expressing EGFP, which was measured by flow cytometry. We conclude that simple complex formation via electrostatic interactions between siRNA and the cationic peptidomimetics is not sufficient for the delivery of siRNA to the RNA interference (RNAi) pathway in the cytoplasm. We are currently testing chemical conjugates of siRNA and the peptidomimetics in concert with liposomal carriers for their ability to enter cells and silence gene expression via the RNAi pathway.