The sealing effect of a few cavity liners upon the dentinal tubule were studied in vitro. The materials employed in this study were Silcot (SP$'{E}$CIALIT$'{E}$S SEPTODONT, M.-T. GENDRAULT, Pharmacien), Hypo-Cal(Ellman Dental Mfg.Co.Inc.), Cavity Lining(De Trey), and Copaute(Harry J. Bosworth Co.). Freshly extracted human teeth were devided into 5 groups by age-under twenty, twenties, thirties, forties, and over fifty. Class V cavities were prepared routinely. The cavity walls of eight teeth of each group were lined by Silcot, Copalite, Cavity Lining, and Hypo-Cal. Remaining eight were not lined as a control. These specimens were immersed in dye solution (2 gm eosin Y to 800 cc distilled water) for 48 hours to allow maximum dye penetration into dentinal tubules. Each specimen was sectioned longitudinally including Class V cavity floor under water spray. Dye penetration into dentinal tubules were examined and following results were obtained. 1. Liners used on this study showed more or less dye penetration into dentinal tubules. But compared with the teeth without lining, the dye penetration of lined specimens were decreased. 2. Of these liners tested, Silcot was the most effective sealer upon the dentinal tubules. Copalite was the moderate sealer and Cavity Lining showed a tendency similar to Copalite. Hypo-Cal revealed the greatest dye penetration. 3. As the age was increased, the more the dye penetration into the dentinal tubules was decreased.