The effects of tetracycline on the subgingival streptococcal flora of periodontal patients were examined. Before antibiotic treatment, tetracycline-resistant isolates were obtained from 24 to 25 patients. In most patients, the proportion of the subgingival flora resistant to tetracycline increased after 2 weeks of therapy (1,000 mg of tetracycline/day) and then decreased after the cessation of treatment. Cultural conditions used for primary isolation were designed to favor the growth of facultative streptococci. Consequently, the majority (99%) of resistant isolates were identified as streptococci. Among 407 tetracycline-resistant Streptococcus isolates chosen for further classification, 9 species were identified, with S. sanguis (63%) and S. mitis (19%) predominating. There were no significant differences in the distribution of species isolated before and after treatment and after the cessation of tetracycline treatment. Plasmids were isolated from only 23 of 121 resistant streptococcal strains examined, suggesting that tetracycline resistance is not plasmid mediated in the majority of these oral streptococci.