The antibacterial effects of chlorhexidine and an oxygen releasing agent on the salivary microflora were evaluated in healthy subjects in two separate experiments: 1) mouthrinsing on a single occasion and 2) mouthrinsing two or four times a day for 1 wk. Both experiments were performed in a cross-over study, and each subject was randomly tested for both rinsing agents and a saline control. The total bacterial number and some selected bacterial genera were followed in saliva samples taken prior to (zero-samples) and on three occasions after the rinsing or rinsing period. The reduction of the total bacterial number by mouthrinsing two or four times a day for 1 wk with chlorhexidine was statistically significant and lasted for over 4 h. After rinsing with chlorhexidine on one single occasion the reduction of the microflora was statistically significant and of the same magnitude as after rinsing two or four times daily for 1 wk. Although statistically significant reduction was obtained, the significant biologic effect of antimicrobial agents may be questioned. The number of Strep. mutans was dramatically reduced with chlorhexidine in some subjects and this reduction lasted for over 1 wk. The oxygen releasing agent did not reduce salivary flora significantly when rinsing took place two or four times daily for 1 wk.