S. SOOKKHEE, M. CHULASIRI AND W. PRACHYABRUED. 2001. The aims of the present study were to screen and characterize the antimicrobial lactic acid bacteria which were isolated from healthy oral cavities of Thai volunteers, and to characterize their inhibiting substances. Among 3790 isolates (suspected to be lactic acid bacteria) from 130 volunteers, five showed an appreciable effect against Sarcina lutea ATCC 9341, Bacillus cereus ATCC 11778, Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 6538, Streptococcus mutans DTMU 1, Strep. salivarius DTMU 1, Strep. sanguis DTMU 1, Candida albicans ATCC 13803 and C. albicans DTMU 2, as well as the oral pathogens. These antimicrobial isolates included L17 and N14 which showed the antibacterial activity, D14 which showed the anticandidal activity, and D6 and N8 which showed both the antibacterial and anticandidal activities. The isolates were later found to be facultative anaerobic, Gram-positive, non-spore-forming, non-capsule-forming and catalase-negative bacilli. They could utilize casein but could not hydrolyse starch, and they produced hydrogen peroxide and bacteriocins. Their antimicrobial potentials were found to be affected by pH, catalase, proteolytic enzymes and temperature. The activity was partially inactivated after catalase treatment, significantly declined at pH ≥9·0 or after trypsin and pepsin treatments, and also reduced after heating at ≥100°C. However, the antimicrobial activity of these five isolates was somewhat resistant to heat. When the isolates were tested for their antimicrobial sensitivity, they were shown to be sensitive to a number of antimicrobial agents. The final identification revealed that D6, D14 and N14 were Lactobacillus paracasei subsp. paracasei, and L17 and N8 were Lact. rhamnosus.