Citric acid (pH = 1) was applied passively or by vigorous burnishing for 3 or 5 minutes to diseased human root surfaces following root planing in vitro. Control surfaces were similarly treated with saline only. Teeth were fixed in glutaraldehyde, dehydrated, freeze-dried, and sputter coated with gold for scanning electron microscopic viewing. Results suggested that optimal exposure of collagen and emineralization occurred with the burnishing application of citric acid for 3 minutes. Although somewhat similar results were found with passive application for 5 minutes, the result of the burnishing application for 5 minutes was that the root was overdemineralized or that the exposed collagen was completely obscured by an amorphous layer that was suggestive of protein denaturization.