The aim of this study was to clarify the antibacterial effect of a mixture of ciprofloxacin, metronidazole and minocycline, with and without the addition of rifampicin, on bacteria taken from infected dentine of root canal walls. The efficacy was also determined against bacteria of carious dentine and infected pulps which may the precursory bacteria of infected root dentine. This efficacy was estimated in vitro by measuring bacterial recovery on BHI-blood agar plates in the presence or absence of the drug combination. Bacteria ranging in number from 10(2) to 10(6) occurred in samples of infected root dentine (27 cases). However, none was recovered from the samples in the presence of the drug combination at concentrations of 25 micrograms ml-1 each. The respective drug alone (10, 25, 50 and 75 micrograms ml-1) substantially decreased the bacterial recovery, but could not kill all the bacteria. Bacteria taken from carious dentine (25 cases) and infected pulps (12 cases) were also sensitive to the drug combination. These results may indicate that the bactericidal efficacy of the drug combination is sufficiently potent to eradicate bacteria from the infected dentine of root canals.