A viable counting technique was used to determine the postantibiotic effect (PAE) of ciprofloxacin against four bacterial species, treated with either one or four times the minimum inhibitory concentration for 1 or 3 h. PAE were demonstrated with Escherichia coli, Staphylococcus aureus or Streptococcus pyogenes after exposure to either concentration for both times. No clear PAE was demonstrated for Klebsiella pneumoniae after any treatment. In some cases, PAE was due to an increased post-treatment lag phase, which was followed by nearly normal multiplication, whereas in other cases a long lag was followed by abnormally rapid cell division, with the generation times of treated cultures being much less than those of the corresponding drug-free controls. This is evidence of recovery of cells that have completed DNA repair. S. aureus, E. coli and K. pneumoniae all exhibited evidence of this type of repair even though K. pneumoniae gave no significant PAE. However, the post-treatment generation times of S. pyogenes, which produced the greatest PAE, gave no evidence of such repair. It is concluded that PAEs may result from a variety of factors.