Class V cavities were prepared in 20 vital teeth and 10 extracted teeth. For all the restorations, glass-ionomer cement was used as a liner at the base of the cavity. The cavity was then restored with a microfilled composite resin. A fter the extraction of the vital teeth, all specimens were sectioned and tested for marginal microinfiltration of 2% methylene blue dye. The teeth restored in vivo showed no signs of infiltration between the dentin and the glass-ionomer liner. The dye penetrated between the liner and the composite resin in five in vivo restorations. The in vitro restorations dye was present between the dentin and the liner. Examination of replicas unde r the scanning electron microscope revealed no gaps between the glass-ionomer cement and the dentin in the in vivo specimens, while gaps of various sizes were present in the in vitro replicas. Glass-ionomer cement was able to bond to dentin when used as a liner in vital teeth but not when used in extracted teeth. This can be attirbuted to the presence of a partially humid environment in vivo that favors the application porcess. In vitro, specimens are to a large extent dehydrated.