The critical etiologic factor in the development of dental caries is dental plaque. The main component of dental plaque is the mutan produced by Streptococcus mutans. The following results were obtained by using blue mutan to assess the factors affecting the mutan-digesting activity of Micromonospora aurantiaca isolated from oral cavity. Micromonospora aurantiaca digested more blue mutan in the minimal essential broth at pH 7.0 than at pH 5.5 or 8.5, and at $37^{circ}C$ than at $32^{circ}C;or;42^{circ}C$. Blue mutan was similarly digested at the range of 1mM to 16mM of $CaCl_2$ and 0.1mM to 6.4 mM of $MgCl_2$, while being significantly digested at the concentration of 2.5mM of KCl. When the concentration of glucose was decreased in the minimal essential broth, the digestion of blue mutan was increased. When the culture supernatant of Micromonospora aurantiaca in the RL broth with 1% glucose or 0.5% mutan was mixed with 2 ${ imes}$ BHIYS broth containing 0.5% yeast extract and 10% sucrose, the formation of artificial plaque on the orthodontic wires by Streptococcus mutans was inhibited(p<0.05). These results indicated that the production of mutanase was identified in the culture supernatant of Micromonospora aurantiaca, suppressing the formation of artificial plaque by Streptococcus mutans.