Abstract –Both chlorhexidine and iodine showed greater antimicrobial effect onin vitrogrownS. mutansplaque than on plaque composed ofS. sanguis. One treatment with iodine for 8 min inhibited the acid production ofS. mutansplaque whereasS. sanguisplaque required 20 min to be similarly affected. In contrast one treatment with chlorhexidine for up to 20 min did not completely inhibit the acid production of plaque of either microorganism. Repeated short term exposures increased the bactericidal effect of chlorhexidine but not that of iodine. The difference in antimicrobial effect between chlorhexidine and that of iodine onS. mutansandS. sanguisshould be investigated in persons heavily infected byS. mutan