Histologic, histometric and morphometric investigations were performed on the gingival tissues of three neutropenic dogs. Over a period of 4 d the animals had been subjected to daily toothcleaning of the left side of both jaws, whereas plaque accumulation and subsequent gingivitis occurred on the uncleaned teeth of the right side of both jaws. Observations and data revealed that subgingival plaque had formed in both sides and extended further apically (max. 0.6 mm) on the cleaned than on the uncleaned sides. In the responding gingival tissues, leukocytes of the junctional epithelium were more numerous and blood vessels of the marginal connective tissue occupied a greater volume on the cleaned sides when compared with the uncleaned sides. It is concluded that neutrophilic granulocytes normally help to prevent bacterial invasion into the dentogingival region, and that, in the presence of subgingival plaque, toothcleaning may aggravate the inflammatory response of the gingiva.