Puprose: The most direct method to clinically evaluate marginal openings in fixed partial dentures is to use manual explorers for tactile perception. The purpose of this in vitro study was to establish the reliability of different instruments for discovering marginal openings. Materials and Methods: The materials used in this study included a dull stylet and a sharp-tipped explorer as well as a metal cylinder surface featuring 9 V -shaped circumferen tial grooves ranging in size from 20 to 165 um. A group of 42 subjects were asked to discover the grooves using both instruments. Results: The data revealed high reliability in the use of the sharp-tipped explorer. With this instrument, a defect of 36 um could be perceived by 95% of the subjects. The dull, worn instrument gave significantly inferior results (P 0.05). Conclusion: Using a sharp-tipped explorer on a smooth metal surface, tactile perception is a reliable means of detecting open margin defects up to 36 um wide. As has been suggested in previous studies, the difficulty seems to be not in defect detection but in the evaluation of the clinical acceptability once it is detected.