Recently, beta-lactam agents containing iron-chelating moieties, such as E0702, which contains catechol, and pirazmonam and U-78,608, which contain 3-hydroxypyridone, have been developed. By determining the susceptibility to these agents of Escherichia coli mutants lacking various iron-repressible outer membrane proteins, we showed that two of these proteins with hitherto unknown functions, Fiu and Cir, were apparently involved in the transport of monomeric catechol and its analogs. These results confirm the conclusion of Curtis and co-workers, which was obtained by using a different set of catechol-containing antibiotics (N. A. C. Curtis, R. L. Eisenstadt, S. J. East, R. J. Cornford, L. A. Walker, and A. J. White, Antimicrob. Agents Chemother. 32:1879-1886, 1988). E0702 was shown to enhance the uptake of radioactive ferric iron into wild-type cells but not into cir fiu double mutants. By combining the influx of E0702 with its hydrolysis by a periplasmic beta-lactamase, we showed that the wild-type cells transported unliganded E0702 at a rate comparable to or even higher than the rate of transport of the E0702-Fe3+ complex. We postulate that the main function of Cir and Fiu may be to recapture the hydrolytic products of enterobactin, such as 2,3-dihydroxybenzoic acid and 2,3-dihydroxybenzoylserine.