PT-01 is a newly developed water-soluble controlled-release insert containing ofloxacin (OFLX) as an antibacterial agent. The effect of PT-01 was evaluated by split-mouth application after oral hygiene instruction. No mechanical debridement was carried out during the experiment. PT-01 was applied once a week for 4 weeks in the periodontal pockets of 46 patients with adult periodontitis. The control insert without any antibacterial agent was applied in a selected pocket in a different quadrant for each patient. Clinical findings plaque index, PI; gingival index, GI; bleeding on probing, BOP; pus discharge, pus; and probing depth, PD at both PT-01 and control sites were measured at every visit for 4 weeks. Clinical findings were improved at both PT-01 and control sites, but the improvements were much more significant at PT-01 sites, in particular BOP and PD, than at control sites. PI was also significantly reduced at PT-01 sites. These results suggest that weekly application of PT-01 in the periodontal pocket has a significant effect on the resolution of periodontal inflammation and has an inhibitory effect on supragingival plaque deposition.