Most older units that drive high-speed dental handpieces include a retraction device designed to prevent dripping of fluids from the water-spray line after the unit has been turned off. Oral fluids and debris may be drawn, along with water, into the water line. Bacteria and debris, unless deliberately flushed out of the water line, may be transmitted to the next patient. Most new units now have mechanisms to prevent retraction. A new dental control unit, designed to eliminate mechanical aspiration of oral fluids, was tested. A kit to test the level of retraction in the water-spray line was also examined. The new unit was found to be effective in protecting the unit water from contamination by oral microorganisms, thereby protecting patients from cross-contamination. However, the control of retraction did not prevent the colonization of the water line by aquatic bacteria. Both internal and external sources of microorganisms must be controlled.