In this investigation, the penetration of different resins into the etched enamel surface and the effect of their viscosity on the penetration of these materials were studied. Freshly extracted teeth were pumiced, rinsed, and dried. The enamel surface was etched by using a solution containing 50% phosphoric acid for one minute; in one group the teeth were etched for two minutes. A layer of enamel surface of another group of teeth was removed before etching; these teeth were etched for one minute. Three different resins--Nuva-Seal, Concise Enamel Bind, and Restodent--were applied to the acid-etched enamel surface for one minute. The Concise composite resin also was applied to the enamel surface of a group of teeth etched for the same period of time. Observations with SEM and polarized light microscopy showed penetration of all the materials into the etched enamel surface to be 5 mum to 10 mum, with a mean of 7 mum. There was no difference in the penetration of these materials despite their different viscosities. The removal of a layer from the enamel surface was not effective in the penetration of the materials.