The present investigation was performed to assess the effect of selective antibiotic therapy on developing plaque and gingivitis in dogs, which at the start of the study had normal gingiva. Fifteen beagle dogs were used. Throughout the entire observation period the animals were fed a diet which favored plaque accumulation. A baseline examination involved assessments of plaque, gingivitis and gingival exudate. The subgingival bacterial flora was assessed by dark-field microscopy. Subsequently the teeth of the right jaws were allowed to accumulate plaque. A careful tooth-cleaning program was maintained in the left jaws. Plaque and gingivitis assessments were repeated and subgingival plaque sampled in the right jaws after 14 and 28 days. On experimental day 28 the second part of the study was initiated. The dogs were randomly distributed into three groups of five animals each. A new baseline examination was performed in the left jaws, after which all tooth cleanings were terminated. During the subsequent 28 days each group of dogs was treated with one of three antimicrobial compounds (vancomycin, metronidazole or clindamycin). Examinations were repeated after 14 and 28 days. The results demonstrated that systemic administration of antimicrobial substances can reduce the rate of plaque formation, change the composition of the developing subgingival microbiota and prevent (or retard) the onset of gingivitis. A comparison of the ability of the three compounds to prevent the formation of a "gingivitis-inducing" plaque revealed that metronidazole and clindamycin were markedly more effective than vancomycin. In fact, in dogs receiving metronidazole and clindamycin treatment, the initiation of gingivitis was almost entirely prevented during the 28 days of treatment.