Polymerization of L-lactide within the pores of anorganic mammalian bone is described. No additional solvent or catalyst is used. The resulting composites exhibit macroscopic morphologies and mechanical properties similar to that of the original bone. We observe an average compressive strength of 194 MPa and an elastic modulus of 8.8 GPa for composites comprised of poly-L-lactide and anorganic bone derived from bovine femurs. Modeling of the reaction kinetics with synthetic sources of crystalline hydroxyapatite powder suggests that polymerization proceeds via a surface-initiated mechanism that is first order in surface area of hydroxyapatite and first order in mole fraction of L-lactide.