This paper presents the results of bursting tests of large, bored, and effectively notched disks removed from long rotor forgings. Most of the tests were conducted at room temperature. For the particular disk geometry employed, the net average tangential stress at bursting speed was as low as 25,000 psi for brittle behaving materials and as high as 77,000 psi for materials approaching ductile behavior. These correspond to 26 per cent and 92 per cent of yield strength, respectively. The Griffith-Irwin theory of crack propagation is adapted to the calculation of Gc, fracture toughness, from notched-disk bursting-test results. It is shown that Gc from disk tests agrees with Gc obtained from slow notched-bend tests, and therefore appears to be a property of material, largely independent of specimen size. The actual magnitude of Gc permits the classification of forgings by the degree of their susceptibility to brittle fracture in the presence of discontinuities. Gc and the ratio Kys of room temperature bursting strength to yield strength are found to correlate well with the material Charpy V-notch fracture appearance transition temperature. As the latter is reduced, Gc and Kys are increased.