Proteus mirabilis, Proteus vulgaris, Morganella morganii, Providencia rettgeri, and Providencia alcalifaciens, which were once classified into the same genus, Proteus, were studied. Cefoxitin-resistant mutants from these species were isolated, and it was confirmed that the resistance was attributed to the lack of an outer membrane protein, resulting in a significant decrease in the penetration of hydrophilic cephalosporins through the outer membrane. Comparison of the mutant strains with their parental strains in the diffusion rates of six monoanionic cephalosporins, a zwitterionic cephalosporin (cephaloridine), and a divalent anionic cephalosporin (cephalosporin C) suggested that each species had only one kind of porin protein, with molecular weights of 40,000 (Proteus mirabilis) or 37,000 (the other four species) and that the porins formed channels with cation selectivity, except for Proteus vulgaris. Porin proteins were purified from all the bacterial species except Providencia alcalifaciens, and the radius of the pores formed by the purified porins was estimated by the use of the liposome swelling assay. The pore radii were estimated to be approximately 0.59 nm (Proteus mirabilis), 0.63 nm (Proteus vulgaris), 0.58 nm (Providencia rettgeri), and 0.60 nm (M. morganii), similar to the size of the pore radius of Escherichia coli porins.