A total of 20 test and 20 control subjects completed an 8-week double blind clinical trial aimed at testing the therapeutic efficacy of a 7% tetrapotassium peroxydiphosphate (PDP) solution administered as a subgingival irrigant to scaled and untreated periodontal pockets. Each patient provided similar contralateral pockets with probing depths at base line of 5 mm or more (mean 6.25 mm). One pocket was selected at random for a thorough scaling, immediately following the base line examination (examination 1). The examination of each experimental site consisted of Plaque Index (P1I) and Gingival Index (GI) scores, probing depth (PD) and attachment level (AL) measurements (in mm), and a differential microscopic proportional count of certain subgingival bacterial morphotypes, namely coccoid cells, motile bacteria (not including spirochetes), spirochetes and all residual bacterial morphotypes. Test patients received coded irrigators containing the active ingredient PDP, while control patients received irrigators containing a placebo rinse similar to the test solution, but without PDP. The participants were instructed to irrigate the test sites twice a day. They were re-examined after four weeks (examination 2) and eight weeks (examination 3) from the base line examination. The results indicated that scaling and irrigation produced a significant decrease from the initial P1I and GI scores, and the PD and AL measurements. The proportions of coccoid cells were significantly increased, while the proportions of motile bacteria and spirochetes were significantly decreased. Irrigation of unscaled pockets resulted in a modest, but significant decrease in the P1I and GI scores.(ABSTRACT TRUNCATED AT 250 WORDS)