The antimicrobial activity of ten root canal sealers was studied on Streptococcus sanguis and Streptococcus mutans. One hundred plates of trypticase soy agar-sheep red blood cell (10%), each with four distinct 3 mm diameter wells, were divided into two groups of 50 by means of the Lawn technique; one group was inoculated with S. sanguis and the other with S. mutans. Four milligrams of each root canal sealer was placed in each of three wells in 10 plates (five plates of each inoculant); the fourth well was left empty as a control site. Plates were incubated at 37 degrees C and checked after 2, 7, and 14 days. All root canal sealers inhibited growth of both organisms. The inhibitory zones for S. sanguis were larger than inhibitory zones for S. mutans for all tested sealers, except Diaket. Dentinol, Kerr, and Tubliseal had significantly more inhibitory effect on S. sanguis than did other tested sealers (p less than 0.05). Diaket had superior inhibition on S. mutans.