Metallurgical examination, including fracture tests, Brinell hardness surveys, chemical analysis, macroscopic examination, and microscopic examination, was made on sections from 2 1/2-in, 3-in, 4-in, and 65-in thick ballistic test plates that had been made from the same hear and had been submitted jointly by Standard Steel Spring Company and Great Lakes Steel Corporation to Aberdeen Proving Ground for experimental and acceptance ballistic tests. The extensive spalling revealed by the ballistic tests was found to have been caused by extremely poor steel soundness. Because of the numerous laminations present in the fractures, the impact toughness of the plates could not be determined accurately. However, fractures of the 4-in and 6-in plates showed much crystallinity, and each plate, except the 3-in plate, was revealed by Brinell hardness surveys and microscopic examination to have been incompletely quench hardened. Grain sizes were coarse (ASTM No. 2 to 4). When evaluating the ballistic limits of at least the 4-in and 6-in plates, their poor impact toughness as well as their poor steel soundness should be considered.