The unnotched Charpy impact properties of carburized 8620, 4620, 4620 + 0.3 Mo, and 3310 were studied at two case depths. The ranking of the alloys in order of increasing impact strength for both case depths was: 8620, 4620, 4620 + 0.3 Mo, 3310. Increasing the nickel content of carburized steel resulted in greater crack initiation resistance in the carburized case. Increasing the molybdenum content, while maintaining the nickel content constant, increased the crack propagation resistance of the core material. Additional tests showed the amount of austenite which transformed prior to fracture and the fracture strain increased with nickel content. The greater amount of austenite transformed during straining before fracture results in the improved crack resistance of carburized cases of the high nickel steels.