The cold-rolled austenitic stainless steels have assumed an important position in missile and space vehicle applications where they are used as skins and other highly stressed structural members at temperatures ranging down to −423 F (the boiling point of hydrogen). The critical nature of structural weight and pressure integrity in pressurized cryogenic propellant tanks in this application requires a material having an optimum combination of high strength-density ratio, weldability, and resistance to brittle fracture at cryogenic temperatures, as well as adequate formability and corrosion resistance. The AISI type 300 series austenitic stainless steels are used for this type of application because of their excellent weldability and toughness at low temperature and their moderately high strength-density ratios which improve markedly at lower temperatures. In this investigation, the mechanical properties of a series of cold-rolled austenitic stainless steels were determined at 78, −100, −320, and −423 F. The alloys were studied in a variety of cold-worked tempers in thicknesses varying between 0.013 and 0.032 in. These alloys were subjected to tension tests in both smooth and notched (stress concentration factor, Kt = 6.3) configurations to provide values of yield and tensile strength, elongation, notched-unnotched tensile ratios in the base metal, and tensile strength and elongation in heliarc butt welded joints. The notched tension tests were included in this study to evaluate toughness which is a measure of resistance to brittle failure. Toughness is a property of vital importance in missile design because these missile structures are subject to shock-type loads which occur during hydraulic hammering, vibration due to rocket engine firing, and action of quick closing valves, and will contain built-in stress concentrations of varying degrees of intensity due to welding defects, tool marks, assembly eccentricities, and random defects in the metal. These conditions all favor brittle failure, and become even more severe at low temperature in that brittle fracture is more prone to occur at reduced temperatures.