SummaryThis study compared the linear leakage patterns shown by 5% aqueous solution of methylene blue (MB) and India ink (II).One‐hundred and twenty‐five roots with single canals from extracted human teeth were used. The canals were cleaned and shaped using the step‐back technique under copious irrigation. Three groups were arranged to receive root fillings of gutta‐percha and one of three proprietary sealers using standard lateral condensation techniques: Sealapex (40 roots), Tubli‐Seal (40 roots) and Ketac‐Endo (20 roots). The remaining roots were used as controls: positive with gutta‐percha and no sealer and negative with the apices sealed with nail varnish. The roots were randomly selected for 7‐day immersion in either MB or II, then sectioned longitudinally; dye penetration was assessed using a Wild Leitz stereomicroscope.In all groups MB showed more penetration than II. The results were statistically significant at theP= 0.01 level, with the exception of the positive control groups. Negative controls showed no penetration.It is concluded that MB, which has a low molecular weight penetrated more deeply along root canal fillings than II, which has a larg