The fracture of seven austenitic stainless steels with varying nickel and nitrogen contents were studied at 4 K. Smooth, 6-mm-diameter tension specimens and 22-mm-thick compact specimens were used. Nitrogen content controlled the yield strength and influenced fracture toughness by its effect on yield strength. Increasing the nickel content increased the fracture toughness at a constant nitrogen content. Observations of the fracture surfaces and polished cross sections through the fracture surfaces of test specimens showed that nucleation controlled the dimpled rupture fracture process. A critical stress criterion for nucleation that depends on both the applied stress and strain was developed and applied to the fracture toughness test. This fracture criterion explained the increase in fracture toughness with increasing nickel and the decrease in fracture toughness with increasing yield strength for strengths over 600 MPa.