Strips containing tetracycline hydrochloride or metronidazole 25% in polyhydroxybutyric acid as a biodegradable polymer matrix, showed sustained release in simulated gingival fluid pH 6.6 at 37 degrees C. When evaluated in patients suffering from advanced periodontal disease, the greatest response to therapy was observed with tetracycline hydrochloride strips inserted into periodontal pockets at four-day intervals for 16 days, compared with an untreated control group. A reduction in plaque index, gingival index and pocket depth was observed. A favourable alteration occurred in the microbial flora of treated pockets with an increase in the proportion of cocci and decrease in gram-negative rods, fusiforms and spirochetes. Metronidazole strips or root-planning tended not to be as effective. The clinical improvement produced by each treatment was not maintained when treatment was terminated.