To study the antimicrobial effect of fluoride-releasing removable orthodontic appliances, 0 wt%(Group 1), 5 wt%(Group 2) and 10 wt%(Group 3) of $CaF_2$ were added to orthodontic resin. The specimens were then put in media in which S. mutans and Lactobacillus, known cariogenic bacteria, were cultured. Inhibiting zones were measured 24, 48 and 72 hours later. The following conclusions were reached: 1 In the Lactobacillus media, no zone of inhibition was observed regardless of the time passed. 2. In the S. mutans media, zones of inhibition were observed after 48 hours. 3. S. mutans showed greater inhibition in Group 3 compared to Group 2, after 72 hours rather than 48 hours(p<0.01).