Tetracycline hydrochloride treatment of cementum and dentin surfaces derived from human teeth not affected by periodontitis resulted in the removal of the smear layer and uncovered a fibrillar collagen substrate. In cementum specimens, the amount of exposure of the organic matrix appeared to be more related to morphologic structure of the cementum and mechanical instrumentation of the root surface rather than to concentration and time of application of tetracycline solution. Time-dependent changes were observed in dentin surfaces, the intertubular matted collagen matrix being evident only in the 4-minute specimens.