Twenty patients were selected for antibiotic treatment due to poor response to conventional therapy. Ten patients were evaluated after taking 250 mg/day of tetracycline for 2 to 7 years. Ten other patients who had been on tetracycline for at least 2 years were evaluated 6 months to 2 years after stopping the antibiotic. Subgingival plaque was cultured anaerobically on nonselective media (ETSA) and ETSA with 1 microgram/ml of tetracycline HCl. Pocket depth, plaque and gingivitis were scored. Those patients on tetracycline had no bleeding on probing despite residual pockets ranging from 3 to 7 mm. Gram-negative anaerobic rods made up to 49.8% of the microflora of these patients, with Fusobacterium nucleatum dominating. B. melaninogenicus, and B. gingivalis were not detected in the samples. Five of ten patients off tetracycline bled on probing and had pocket depths in the same range as those on tetracycline. The microflora of tetracycline-off sites was predominately Gram-negative rods (63.1% with B. gingivalis and F. nucleatum 7.3% and 3.1% of the flora respectively). In tetracycline-on samples 76.6% of the isolates were resistant to 1 microgram/ml of tetracycline compared to 25.9% in the patients off tetracycline and 7.1% resistant organisms in 14 untreated control samples from periodontitis patients not exposed to any long-term tetracycline therapy. Long-term, low-dose tetracycline was associated with a healthy clinical condition and diverse Gram-negative anaerobic flora resistant to the antibiotic. After discontinuing tetracycline the clinical and bacterial status was more characteristic of disease.