Clinical orthopaedic use of calcium phosphate cement has been limited due to its slow resorption rate, but a new macroporous a-tricalcium phosphate (a-TCP) bone cement has been designed to accelerate resorption and to increase bone ingrowth. To assess its clinical potential, the in vivo behaviour of a-TCP was evaluated in a critical-size defect drilled in the femoral condyles of 36 adult female New Zealand rabbits. Macroporous or standard cement was injected immediately after preparation of the defect. The foaming agent was albumen, which gave up to 75% porosity. The rabbits were divided into three groups and the lesions examined histopathologically at 1, 4 and 12 weeks. No inflammatory reaction was detected at any time period following implantation with either macroporous or standard cement. At 12 weeks, the area of the implanted macroporous cement was approximately 35% of the initial lesion size. Bone growth and revascularisation was observed inside the central pores of the macroporous cement, not only at the margins, as was found with standard calcium phosphate cement. The results indicated that both cements were osteoconductive, biocompatible and biodegradable but their different physicochemical and biological properties had a marked influence on their post-implant behaviour.