The susceptibility of Porphyromonas gingivalis to amoxicillin, doxycycline and metronidazole was determined by a standardized method taking into account the biofilm mode of growth of subgingival bacteria. The minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of 48-h biofilms of P. gingivalis established on membrane filters in a Modified Robbins Device were determined by agar dilution. The results were compared to (i) conventional MIC determinations, (ii) the susceptibility of planktonic cultures with cell numbers equal to those of the biofilms, and (iii) results for detached biofilm cells. The MICs of the biofilms of the six reference strains and clinical isolates containing 107-8 cells/filter were much higher than the conventional MIC values. However, the MIC of planktonic cultures of equal cell numbers also increased, indicating that an inoculum effect is part of the explanation of the increased resistance of biofilms. Still, the MBCs of biofilms were 2-8 times, and those for doxycycline up to 64 times, greater than the MBC values for planktonic cultures.