This study evaluated the ability of ultrasonics and calcium hydroxide to remove pulp tissue debris from the mesial root canals of human mandibular molars. All teeth were instrumented using a standard filing technique and irrigated with an equal volume of 2.6% sodium hypochlorite before the application of the experimental debridement methods. Debridement comparisons were made of both instrumented and uninstrumented controls at the 3-mm and 1-mm levels of the canals and isthmuses. Statistical analysis showed no differences among the experimental groups or the instrumented controls in the canals at either level or isthmuses at the 3-mm level. In the isthmuses at the 1-mm level, no differences were found among the experimental groups, but they were all significantly cleaner than the instrumented controls. These results indicate that calcium hydroxide and ultrasonics are equally effective in debriding the root canal system, and that both are significantly better than standard instrumentation alone in the isthmuses at the 1-mm level.