Subinhibitory concentrations of some antibiotics are able to inhibit adhesion of bacteria to human host cells, to facilitate phagocytosis and to modify the shape of the bacteria cell wall, e.g., variable degrees of filamentation occur frequently in gram-negative bacteria. The kinetics of filamentation of Escherichia coli were investigated by incubation for various periods up to 18 h, with different subinhibitory concentrations of ceftibuten, from 1/2 to 1/128 of the MIC, corresponding to 0.25-0.003 micrograms/ml. Normal shapes, short and long filamentation and bacterial ghosts were observed. The morphological changes in the bacterial cells were influenced by the duration of exposure and by the antibiotic concentration. The greatest filamentation did not occur at 1/2 MIC, the concentration of ceftibuten closest to the MIC, but at 1/8 MIC, and filamentation plus ghosts were maximal between 8 and 18 h of incubation. The morphological changes observed clearly show that ceftibuten has a greater affinity for and impairs the function of penicillin-binding protein 3 (involved in synthesis of peptidoglycan for cross walls) more than other cephalosporins, such as cephaloridine or cefoxitin.