White spot lesion is observed in nearly 50% of patients undergoing orthodontic treatment. Long lasting antibacterial properties of orthodontic cements may reduce this phenomenon. The antibacterial properties of four orthodontic cements were evaluated using direct contact test (DCT) and agar diffusion test (ADT). Using the DCT technique, octet samples of glass ionomer (CX-Plus, SHOFU Inc), reinforced glass ionomer (GC Fuji ORTHO LC, GC Corporation Inc), and two composite (Transbond XT, and Transbond Plus, 3M Unitek) orthodontic cements were placed on the sidewalls of wells of a 96-microtiter plate. S. mutans cells (ca. 1X106) were placed on the surface of each sample for 1 h at 37°C. Then, fresh media was added to each well and bacterial growth was followed for 16 h by temperature-controlled spectrophotometer. This was repeated on samples aged in phosphate buffer saline for 2 weeks and 1 month. ADT was performed by placing samples in wells punched in agar plates. Measuring the halo in bacterial lawn, after 48 h, of the 4 cements tested, only glass ionomer cement demonstrated a limited inhibition zone (1.2 mm). Results in the DCT showed that only reinforced glass ionomer exhibited potent antibacterial activity that lasted 2 weeks and faded within the next 2 weeks. The results obtained by the agar diffusion test cannot be compared to those obtained by direct contact test and the reinforced glass ionomer cement possessed the most potent antibacterial activity by direct contact test.