During a 5 day period without oral hygiene, 4 groups each consisting of 3 subjects with healthy gingivae, rinsed their mouths with solutions of tetracycline, vancomycin, polymyxin B or distilled water. Gingival plaque accumulated rapidly in the group rinsing with water, while tetracycline, and to a lesser degree vancomycin and polymyxin B inhibited plaque formation. Clinical gingivitis was not observed in any of the participants, but gingival exudate and leukocyte emigration were noted and seen to increase during the experimental period, especially in the water group. Rinsing with antibiotics inhibited these signs of subclinical inflammation to varying degrees. The bacterial composition of the gingival plaque changed in the water group during the experimental period towards a higher percentage of gram-negative bacteria, following the pattern observed in previous investigations. Tetracycline markedly reduced the numbers of gingival plaque organisms, while polymyxin B favored a proliferation of gram-positive cocci and short rods and depressed gram-negative bacteria. Rinsing with vancomycin resulted in a pronounced shift towards an almost pure gram-negative plaque flora. It is concluded that local administration of antibiotics with a limited spectrum may be useful for the study of the pathogenicity of the different components of the gingival microbial flora.