The purpose of this study was to an in estimate the antibiotic susceptibility of P. gingivalis and P. interrnedia isolate from the subgingival plaque to adult periodontitis. Six P. gingivalis and five P. intermedia bacterial strains were tested for their susceptibility to 10 antimicrobial agents under disc diffusion method and broth dilution method. Ten patients with deep pocket(6mm) were selected for this study. They had not taken antibiotics for 6 months and no history of dental treatment for 6 months before this study. The result were as follow : 1. For antibiotic disc diffusion method, six P. gingivalis and five P. interrnedia were tested with 10 antimicrobial agents which comprised penicillin, gentamycin, clindamycin, lincomycin, ampicillin, erythromycin, tetracycline, amikacin, chloramphenicol, vancomycin. The sensitive antibiotics were tetracycline, penicillin, ampicillin, vancomycin, chloramphenicol and resisitent antibiotics were lincomycin. The other antimicrobial agents were less active. 2. From the study of determination on the minimal inhibitory concentration(MIC) by broth dilution method, the MIC of tetracycline to P. gingivalis and P. intermedia were $0.5-1.0{mu}g/ml$, $0.5{mu}g/ml$, that of ampicillin were $1-8{mu}g/ml$, that of clindamycin were $1-32{mu]g/ml$, $8-16{mu}g/ml$, that of lincomycin were $16-32{mu}g/ml$, $2-32{mu}g/ml$. These data suggest that tetracycline and ampicillin may be valuable drug in the elemination of P. gingivalis and P. interrnedia from the patients with adult periodontitis.