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 mL) 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 37oC 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.