The plane-strain fracture toughness, KIC, defined by ASTM E 399, is assumed to represent a size insensitive lower bound value. The interpretation is due to the original work by George Irwin. In this work the consistency of the ASTM KIC plane-strain fracture toughness standard (ASTM E 399) is examined by reassessing the original data used to develop the standard, based on present knowledge about fracture micromechanisms. Originally, the standard was based on continuum mechanics assumptions, which have later been found inadequate to describe the real physical fracture process. The materials used for the development of ASTM E 399 were generally aluminum and titanium alloys or extra high strength steels. The materials had in common that their fracture micro-mechanism was ductile fracture, i.e., the materials showed a rising tearing resistance curve. Therefore, the fracture toughness did not show the expected decreasing trend with increasing specimen size, but generally the opposite trend. The specimen thickness was assumed to be the limiting dimension, even though much of the experimental data indicated that the specimen ligament size, not the thickness, controlled the fracture toughness value.