The subgingival microbiologic composition of diseased periodontal sites was evaluated by darkfield microscopy before and after scaling or local delivery of tetracycline. A standardized sampling and counting method using a crevicular washing technique was developed to determine both numbers and proportions of morphotypes using darkfield microscopy. Tetracycline-loaded hollow fibers established an initial intrasulcular concentration of 200,000 micrograms/ml, which decreased exponentially to 15 micrograms/ml in 24 hours. Repetitive intrasulcular placement of these fibers at periodontitis sites produced an incremental reduction in bacterial counts over a 10-day period. Monolithic fibers made of ethylene vinyl acetate loaded with 25% tetracycline hydrochloride provided sustained release for 10 days under in vitro test conditions. Ten patients were treated in a study comparing the effects of these fibers with scaling. Fibers were placed subgingivally to fill pockets to their probable depth and covered with a periodontal dressing which was maintained for 10 days. The average intrasulcular tetracycline concentration measured at the end of the 10-day period was 643 micrograms/ml. At these sites, total counts, spirochetes, motile rods and nonmotile rods were significantly reduced immediately following treatment. Total counts were depressed to levels near the detection limit of darkfield microscopy. In comparison, scaling produced much smaller alterations of darkfield counts which were not statistically significant.