Scanning electron microscopy was used to study the formation of a dentine smear in ten premolars ground with a water-cooled diamond in an air turbine. In another ten teeth the pulp reaction in full crown preparation with the same armamentarium was investigated by histological techniques. The findings revealed the formation of a dentine smear which was easily removed by light polishing with wet pumice, leaving plugs of debris in the tubule apertures. The clinical significance of these findings needs further investigation. No bacteria were demonstrated on the prepared surfaces either in the scanning electron micrographs or histologically when stained with Brown & Brenn stain. Severe, acute pulp reactions were observed subjacent to the dentinal tubules cut in full crown preparation. This technique should therefore be limited to superficial use only.