The potential use of an adjunctive therapy of metronidazole plus amoxycillin for the subgingival elimination of Actinobacillus actinomycetemcomitans in periodontitis patients was investigated. 22 patients participated in this study, 11 with localized juvenile periodontitis (LJP) and 11 with rapidly progressive periodontitis (RPP). 14 patients had received periodontal treatment in the past. All patients were subgingivally infected with A. actinomycetemcomitans. After mechanical subgingival debridement in combination with the antibiotic treatment, elimination of A. actinomycetemcomitans was achieved in all patients but one. With this one exception, clinical improvements were observed in all patients, resulting in reduced pocket probing depths as well as in a significant reduction in bleeding on probing. Re-examination of 16 patients after 9-11 months revealed that A. actinomycetemcomitans was still undetectable and further clinical improvement was observed. It was concluded that the combination of metronidazole plus amoxycillin is a valuable adjunct to mechanical therapy in A. actinomycetemcomitans associated periodontal infections.