Carburized SAE 8620 and 4820 steels were subjected to bending fatigue testing with systematic changes in applied stress. Fracture toughness of the high-carbon martensite/retained austenite case microstructure of carburized specimens was determined by the measurement of crack sizes associated with unstable intergranular crack propagation as a function of applied stress. Fracture toughness varied between 11 and 22 MPa m(1/2) and increased with increased applied stress. The results are discussed relative to strain-induced martensitic transformation of retained austenite as a function of applied stress.