Basic fuchsin has been established as an effective testing agent in determining the extent of microleakage present under restorations. Recently, the presence of microleakage in vivo has been studied through the use of hydroxyl ions as is present in proprietary cavity liners such as Dycal. The purpose of this study was to determine a correlation between the use of these two systems in measuring microleakage. A series of restored human teeth containing a calcium hydroxide liner were thermocycled and then tested with pH indicating paper. The same samples were then given additional thermocycles in 0.5% basic fuchsin dye. The teeth were sectioned in two planes and ranked as to the amount of dye penetration. Over an 18-week period the number of positive pH tests began to diminish while the dye penetration values remained relatively constant. The dye penetration method continued to record microleakage after the pH paper had ceased to detect hydroxyl ions. The dye penetration method appeared to be more dependable as the age of the restoration increased.