The present study identified the properties of porins in the outer membrane in Pseudomonas aeruginosa,and showed the role of outer membrane in determining imipenem diffusion in Pseudomonas aeruginosa. The molecular weight of the major outer membrane protein was analyzed by SDS-PAGE. The purification of the porins in Pseudomonas aeruginosa was achieved by DEAE ion - exchange HPLC. The purified outer membrane proteins were reconstituted with phosphatidylcholine and dicetylphosphate into membrane vesicles, and were tested by the liposomes swelling method for the diffusion of imipenem. The permeability assay showed that OprC (70 kD), OprD2 (46kD), and OprE(43 kD) were the channel- forming proteins. But only OprD2 was thought to be the likely route of imipenem diffusion.