The bioactive glass known as Bioglass or Perioglass (USB) (US Biomaterials, Alachua, FL) has proven to be an effective graft material owing to the apatite layer which forms on the surface of the glass, promoting bone formation. USB particles range in size from 90 to 710 microns in diameter, as determined by optical microscopy. A similar bioactive material, BioGran (OV) (Orthovita, Malvern, PA), was developed to limit the particle size of 4555 to the range between 300 and 360 microns, as determined by sieving. The objective of this study was to histologically and biomechanically compare the 4555 bioactive glass, produced by US Biomaterials, in a wide particle range (USB) to the narrower particle range glass produced by Orthovita (OV) The grafted defects will then be compared to normal cancellous bone (NORM) of the distal femur in rabbits. Histologically, more bone was quantified at both 4 and 12 weeks within the defects filled with USB and NORM when compared to the limbs filled with OV (p < 0.05). The OV particles had greater particle axes and larger particle areas on average than the USB particles (p < 0.05). However, the particle axis and area of the two materials decreased with time at a similar rate. Biomechanically, the USB- and OV-grafted defects had comparable peak compressive load, compressive stiffness, and compressive modulus which were equivalent to normal bone.