The report compares the mechanical properties of air-melted versus three different types of vacuum-melted AISI 4340 steel. Cleanliness, hardenability, tensile, impact, fatigue properties and fracture toughness characteristics are compared at two strength levels (200-220 ksi and 260-280 ksi). Particular emphasis was placed on transverse mechanical properties. Susceptibilities to hydrogen embrittlement of the variously different processed materials are also compared. In general, the data indicate that vacuum melting offers a significant improvement in mechanical properties over air-melted steel in the transverse grain direction. In particular, the fracture toughness, impact strength, fatigue, ductility and resistance to hydrogen embrittlement are significantly enhanced by the vacuum melting process.