This study investigated the effect of citric acid application on periodontally involved root surfaces. Forty periodontally involved teeth were randomly divided into four groups of ten teeth each: no treatment, citric acid treatment, root planing alone, and root planing in conjunction with citric acid treatment. Ten nondiseased, untreated teeth served as controls. After treatment, each tooth was split along its long axis; half was examined under light microscopy and half under scanning electron microscopy (part II of the paper). Light microscopy revealed that the effects obtained by scaling and root planing were not altered after citric acid application. Moreover, the cementum layer was not entirely removed without careful and thorough planing of the root surface. Citric acid application alone had no effect on the diseased root surface. Citric acid did not penetrate the dentinal tubules, nor did it alter the collagen content of the roots obtained by scaling and root planing.