The purpose of this in vivo study was to investigate the antimicrobial effectiveness of 6% sodium hypochlorite, when used with the conventional irrigation technique using a 30 gauge side-vented needle, the EndoVacTM System, and the ProUltra PiezoFlowTM Ultrasonic Irrigation Needle. It was hypothesized that both the EndoVacTM system and the PiezoFlowTM Ultrasonic Irrigation Needle would remove bacteria from root canals more effectively than irrigation using a 30 gauge side-vented needle. After obtaining informed consent from each subject, teeth were randomly divided into three treatment groups (n=30), and samples were collected following instrumentation and irrigation using one of the three irrigation techniques. The samples were cultured under aerobic and anaerobic conditions, and bacteria growth was measured by direct counting of colonies and grid specific calculations. Conclusions: The data indicated that the 30 gauge side-vented needle and the EndoVacTM System removed significantly more bacteria than the ProUltra PiezoFlowTM Ultrasonic Irrigation Needle.