Polyhydroxybutyrate (PHB) is a polyester made by many microorganisms under conditions of nitrogen deficiency, and is produced commercially in bulk by biotechnology. It has been suggested that PHB-based materials (copolymers and composites) could be suitable for medical applications and may be biodegradable. This paper presents some findings regarding the degradation and biological properties of polyhydroxybutyrate and composites reinforced with particulate hydroxyapatite. It has been established that the strength and stiffness of these materials reduce on in-vitro environment exposure in phosphate-buffered saline at 37 degrees C for periods up to 4 months, and that the degradation rate is a function of composition and processing conditions. It has also been demonstrated that materials based on PHB produce a consistent favourable bone tissue adaptation response with no evidence of an undesirable chronic inflammatory response after implantation periods up to 12 months. Bone is rapidly formed close to the material and subsequently becomes highly organized, with up to 80% of the implant surface lying in direct apposition to new bone. The materials showed no conclusive evidence of extensive structural breakdown in vivo during the implantation period of the study.