Penetration of bacteria into the tubules of intact dentin exposed by fracture was comparedin vitroin pairs of teeth, one of which in each pair was mounted with an intrapulpal hydrostatic pressure equivalent to 30 mm Hg. The teeth were incubated at 37°C for 21 days. Intra-pair comparisonsin vivowere made of bacterial invasion into dentinal tubules beneath ground dentin surfaces and beneath fractured or acid-treated surfaces, which were exposed for 1 week. It was found that an outward fluid flow in the dentinal tubules due to intrapulpal pressure may mechanically hinder bacterial growth into the tubules. Of greater importance as an obstruction to bacterial invasion was the blocking of the outer apertures of the dentinal tubules by grinding debris. This barrier, however, seemed to be removed after some days and this would allow bacterial ingrowth into intact vital dentin. It is concluded that dentinal surfaces that have been left unprotected and are covered with plaque for some days should be treated as carious dentin, and the surface layer should be ground off to remove widened apertures filled with bacteria.