To compare the effectiveness of different irrigation procedures on the removal of the smear layer, 40 curved canals were manually prepared in vitro under copious irrigation with 3 per cent NaOCl. Ten canals served as controls. The remaining 30 were equally distributed in three experimental groups and subsequently irrigated with the following: ultrasound with NaOCl; EDTA; ultrasound with EDTA. In the scanning electron microscope, the appearance of the smear layer was rated over the distribution of dentinal tubule openings for the coronal, middle and apical segments of the canals. Irrigation with NaOCl produced consistently smeared surfaces. Ultrasonic stirring of NaOCl removed the smear layer moderately, while EDTA produced almost smear-free surfaces. Ultrasound in association with EDTA did not enhance the dissolving capability of this chelating agent. A definite decline in the efficiency of the irrigation procedures was also observed along the apical part of the canals.