This study was done to find out the influence of three different vehicles on the antimicrobial activity of calcium hydroxide against the four microorganisms commonly seen in the infected pulp of the root canals. For this study cell culture plates with solid media were used. The results of the study showed that all calcium hydroxide pastes were effective in killing the microorganisms tested but at different times. Amongst them only calcium hydroxide CMCP pastes and glycerin pastes were the most effective against the microorganisms tested.