A study was carried out to compare the effects of subgingival applications of resorbable hydroxypropylmethylcellulose (HPMC) membranes containing 20% chlorhexidine (CHx) and 40% doxycycline (DOXY) placed subgingivally in periodontal pockets greater than 5 mm in depth. Each of 11 patients had at least four pockets. HPMC strips containing CHx and DOXY were inserted into each pocket twice a week for 6 w. One pocket was kept as a control and the other received subgingival scaling and root planing at 0, 1, 3 and 6 w. The microbial flora of 44 pockets from the 11 patients was examined using dark-field microscopy at the baseline and at 1, 3, 6 and 10 w. Clinical parameters including probing depth, crevicular fluid flow, bleeding on probing and gingival shrinkage were also recorded. The CHx and DOXY-administered or root-planed pockets showed marked reduction of both pocket depth and the number of motile organisms. Less bleeding was also observed in CHx and DOXY-inserted pockets.