A novel method was developed to quantitatively determine the membrane transduction efficiency of cationic oligopeptides, allowing for selective measurement of transduction, excluding concurrent adsorptive endocytosis. It was found that Tat-(47-57) (YGRKKRRQRRR) YG(1-R)(9), and guanidinated-YG(1-K)(9) were preferentially transduced to the cytosolic compartment, while YG(1-K)(9) was primarily endocytosed. Studies of various oligoarginine peptides (4-15 residues) demonstrated that internalization through transduction remained constant, while the amount internalized via endocytosis increased with arginine length, indicating that oligopeptide transduction requires the guanidine structure of arginine, while endocytosis depends only on the number of positive charges. The addition of unlabeled-YG(1-R)(9) dose-dependently inhibited transduction of I-125-labeled-YG(1-R)(9), while endocytosis remained constant. Unlabeled-YG(d-R)(9) and Tat-(47-57) were able to inhibit transduction similarly, while YG(1-K)(9) had no effect. These studies demonstrate specific surface binding sites required for transduction of oligoarginine, but not for endocytosis. Data generated from this study are essential to elucidate the transduction mechanism and to develop an efficient carrier system for cytoplasmic drug delivery. (C) 2003 Elsevier Science (USA). All rights reserved.