Protein D2 of Pseudomonas aeruginosa outer membrane is known to facilitate the specific permeation of imipenem (N-formimdoylthienamycin) across this membrane barrier. We have characterized the binding site in the protein D2 channel by studying the competitive inhibition, by various solutes, of imipenem diffusion into the periplasm. We found that basic amino acids, lysine, arginine, histidine, and ornithine, were effective inhibitors. L- and D-lysine were found to be competitive inhibitors with approximate Ki values of 0.6 and 0.3 mM, respectively. Peptides containing L-lysine at the carboxyl terminus, as well as dipeptides containing L-lysine at the amino terminus, were also able to inhibit the transport. Wild type cells transported tripeptide Thr-Ser-Lys into the periplasm three to four times as rapidly as the mutant cells lacking the D2 protein. These results suggest that protein D2 plays a physiologically significant role in the uptake of basic amino acids and peptides containing these amino acids across the outer membrane of P. aeruginosa.