INTRODUCTION: The purpose of our study was to evaluate the antibacterial effect of ProRoot MTA (PRMTA), Root MTA (RMTA) and Portland cement (PC) at their clinical concentration (70 mg/25 µL) against Actinobacillus actinomycetemcomitans (Aa) one of the prominent periodontal (pocket) microorganisms. MATERIALS AND METHODS: Agar diffusion test on Blood Agar with Hemin and Vitamin K (BAHV) was employed in this study. The microorganisms were seeded on the BAHV by spreaders. Small holes, 6 mm in diameter, were made in the BAHV by removing agar. PRMTA, RMTA and PC were placed into the wells immediately after manipulation. The plates were incubated in anaerobic atmosphere at 37°C for 72 h and the zones of inhibition were measured. RESULTS: In the agar diffusion test PRMTA, RMTA and PC against Aa showed zones of inhibition. Analyzing the antimicrobial activity of PRMTA, RMTA and PC according to paired one-way ANOVA and Post Hoc Test (Turkey's test) analysis showed a statistically significant difference (P<0.05) between PRMTA, RMTA and PC. RMTA showed the largest zone of inhibition (29 mm) against Aa. There was no difference in the zones of inhibition between the 48 and 72 h time periods. CONCLUSION: In this in vitro study PRMTA and RMTA presented similar antimicrobial activity against Aa.