Recently, electrolyzed acidic jell was developed by the electrolysis of glycerin aqueous solution containing sodium chloride using a special device. This jell has a wound-healing activity and a strong bactericidal effect on anaerobic gram-negative obligate rods on one hand, it is reported to induce no resistant bacteria on the other hand. In the present study, for the purpose of clinically applying electrolyzed acidic jell to local delivery into periodontal pockets for treating periodontal diseases, we investigated its bactericidal potency in vitro in the four strains of the two species of gram-negative obligate anaerobic rods, Porphomonas gingivalis and Fusobacterium nucleatum, known to have strong periodontal pathogenicity, and administered the jell into periodontal pockets and investigated effects on clinical symptoms and bacteria and black pigment-producing anaerobic rods in periodontal pockets. In vitro, soft-type electrolyzed acidic jell (without grape) showed a bactericidal in vivo effect on all the four bacterial strains tested immediately after application, whereas hard-type electrolyzed acid jell (with grape) began to show a bactericidal effect on two standard bacterial strains 10 min after application and on clinically isolated two bacterial strains immediately after application. Hard-type electrolyzed acid jell (without grape) was bactericidal immediately after application to the clinically isolated P. gingivalis and began to show a bactericidal effect on other three bacterial strains 10 min after application. In vivo, soft-type electrolyzed acidic jell (without grape) significantly reduced the number of bacteria 1, 3 and 7 days after administration into periodontal pockets compared with immediately before administration, and this effect was nearly equipotent to that of Periofeel®. The number of bacteria was also decreased by other three types of jell, though not significantly. No significant change was observed in clinical symptoms before and after jell administration.<BR>These results demonstrate that electrolyzed acidic jell has a potent bactericidal activity to gram-negative obligate anaerobic rods and reduces bacteria inside periodontal pockets.