PT‐01is a newly developed water‐solublecontrolled‐release insert containing ofloxacin (OFLX) as an antibacterial agent. The effect of PT‐01 was evaluated by splitmouth 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.J Periodontol 1992; 63:2–6.