The present study describes results on selected clinical and microbiological parameters obtained by periodontal treatment with ethylene vinyl acetate fibers containing 25% by weight tetracycline hydrochloride placed into the periodontal pocket alone or in combination with scaling. Supragingival plaque control was maintained throughout the study by weekly professional cleaning and 0.2% chlorhexidine mouthrinses. Controls included untreated sites and sites treated by conventional scaling alone in a 4-quadrant split-mouth design. The experiment was conducted on 95 teeth from 10 subjects with periodontal pockets greater than or equal to 6 mm which initially bled on probing. All treatments resulted in changes indicative of effective therapy. Pocket depth was reduced, bleeding on probing decreased and gingival index scores decreased. Parallel to the clinical changes, all treatments reduced total bacterial numbers, % black-pigmented Bacteroides, motile bacteria, non-motile rods, and produced a proportionate increase in cocci. Fiber therapy with or without scaling reduced bacterial counts by approximately 2 orders of magnitude when evaluated at 62 days post-therapy. The combination of fiber therapy with scaling was particularly effective, suggesting a possible synergy between these forms of therapy. The combined therapy eliminated bleeding on probing, and black-pigmented Bacteroides, and produced the greatest mean reduction in pocket depth.