Summary Nine root canal cements were tested for their antimicrobial action; the obligate anaerobe B fragilis was used as an indicator. Wells were prepared in agar plates inoculated with the test organism. The wells were filled with each of the freshly mixed cements. After incubation, the zones of inhibition of bacterial growth were observed and measured. AH-26, N 2, Proco-Sol, Roth, and ZOE cements caused significant inhibition of growth; Diaket, Kerr, and Tubliseal cements resulted in inhibition of growth to a moderate degree, and Mynol cement was not inhibitory against B fragilis.