In this study, Knoop hardness experiments on artificial carious enamel (pH 5.0, 4.5 and 4.0) are presented with the load perpendicular to the subsurface lesion. The results show that for human and bovine enamel the indentation length is proportional to the square root of the load. The proportionality constant is numerically estimated for all pH values for human and bovine enamel. The indentation measurements reflect the decalcification despite the presence of surface layer covering the lesion.