Actinobacillus actinomycetemcomitans is a Gram-negative oral microorganism, which has been implicated in the etiology of localized juvenile periodontitis and in severe medical infections such as bacterial endocarditis. This study evaluated the ability of periodontal probes to transmit A actinomycetemcomitans from juvenile periodontitis lesions to healthy gingival sulci in the same patient. Localized juvenile periodontitis patients exhibiting first molar and incisor alveolar bone loss and with large numbers of A actinomycetemcomitans in deep periodontal pockets were included in this study. A periodontal probe was inserted into periodontal pockets of 6 mm or greater depth. The probe was then placed into a healthy gingival sulcus of 3 mm or less, in the same subject. Fifty-five transfers by probing were made and A actinomycetemcomitans in both the donor and recipient sites was assessed by a selective culture technique. The results indicate that periodontal probes can become contaminated with A actinomycetemcomitans from juvenile periodontitis lesions during routine dental examinations and can transfer this microorganism from infected to previously uninfected sites. However, A actinomycetemcomitans inoculated into the healthy gingival sulci did not permanently colonize these sites since the organisms were eliminated within 3 weeks.