A library of isogenic mutants containing Mud9-induced deletions of the structural and regulatory genes for the porin proteins OmpF and OmpC of Escherichia coli was constructed. The accumulation of norfloxacin, tetracycline, chloramphenicol, cephalothin and cefoxitin was measured with each strain, and shown to be reproducible with low experimental standard deviations, such that the roles of OmpF, OmpC and PhoE in the accumulation of these agents were determined. All data were statistically analysed to determine whether the differences observed between the data for each mutant compared with those for the other mutants and for the wild-type strain were significant. The loss of OmpF reduced accumulation of norfloxacin, tetracycline, cephalothin and cefoxitin by 16-60% compared to the wild-type parent strain, but reduced accumulation of chloramphenicol by < 10%. The loss of OmpC reduced accumulation of cephalothin and cefoxitin by 13 and 34%, respectively, compared to the wild-type parent strain, but had little effect on the accumulation of norfloxacin, chloramphenicol and tetracycline (< 3%). The loss of both OmpF and OmpC (ompR) reduced accumulation of norfloxacin, chloramphenicol, tetracycline, cephalothin and cefoxitin by 36-68%. However, the presence of PhoE in the absence of both OmpF and OmpC, enhanced accumulation to 52-119% of the concentrations of these five agents accumulated by the wild-type strain. These data suggest that OmpF is the preferred route of entry for three of the antibiotics studied, but not for chloramphenicol and tetracycline which utilize both porins equally well. The high levels of accumulation (30-64%) of all five antibiotics in the absence of all major porins suggest that an alternative mechanism(s) of accumulation is available.