Determination of the rates of diffusion of beta-lactam antibiotics through purified Pseudomonas aeruginosa porins C, D2, and E in liposomes yielded the following results. (i) The rates of carbapenem (imipenem and meropenem) diffusion through the protein D2 pore were roughly 2 to 70 times higher than those through other porin pores. It is not clear why the protein D2 pore allowed rapid diffusion of carbapenems. The rates of diffusion of glucosamine and triglycine through the protein D2 pore were about 14 and 4 times higher, respectively, than that of an uncharged test solute with a similar Mr, glucose. (ii) The rates of diffusion of antipseudomonal anionic beta-lactams such as piperacillin, ceftazidime, cefsulodin, and aztreonam through the protein C pore were higher than those through other porin pores. This was probably due to the slightly larger pore size and the slight anion selectivity of protein C, since the apparent exclusion limit of the protein C pore for uncharged saccharides is higher than that of other porins and the rate of diffusion of gluconic acid through the protein C pore is about double that for glucose. (iii) The rates of diffusion of cefoperazone through all three species of porin were relatively high. These results indicate that the antipseudomonal beta-lactams permeate the P. aeruginosa outer membrane via newly identified porins.