The circumferentially cracked bar (CCB) specimen has been developed as an alternative geometry for fracture toughness testing. A Test Procedure has been developed to formalise the use of CCB specimens for fracture toughness testing. To validate this Procedure for the assessment of ductile initiation toughness, a series of CCB tests has been carried out on a well-characterised A533B-1 steel plate. Supporting finite element analyses were performed to provide accurate η-factors for the derivation of J from load-displacement traces. Fatigue pre-cracking of specimens in rotating bend provided concentric cracks of crack depth to specimen radius ratios recommended by the Procedure. Resultant initiation toughness values, defined at 0.2 mm of ductile crack extension including blunting, were higher than the corresponding value derived using three-point bend specimens, and lower than that obtained from centre-cracked tension specimens. This ranking of initiation toughness was consistent with that expected from a consideration of crack-tip constraint. This suggests that the CCB specimen may be useful in assessing constraint effects on fracture at moderately low constraint levels.