A review of the literature demonstrated that controversy exists as to whether anesthesia circle systems can be a source of transmission of bacteria to patients. The purpose of this study was to evaluate the use of one airflow filter (on the expiratory limb) versus two airflow filters (one on the inspiratory limb and one on the expiratory limb) in the prevention of bacterial migration in the anesthesia circle system, during general anesthesia, using semiquantitative analysis of the number of bacterial colony forming units (CFUs). This study consisted of two randomized groups receiving one or two airflow filters. Thirty-five volunteers participated in the study. At the conclusion of the surgical cases, a culture was taken of the anesthesia circle system. The cultures were read at 24 and 48 hours for the presence of CFUs. No CFUs were reported in either group. The results of the study were inconclusive. Therefore, the use of one airflow filter on the expiratory limb should remain the standard of care, pending additional research. Some practitioners may use a second airflow filter when there is a greater risk of infection, such as in the case of an immunocompromised patient. However, this study can neither support nor question the effectiveness of this practice.