We report on the development and characterization of a new composite material consisting of amorphous carbonated apatite, Ca₅(PO₄,CO₃)₃(OH), and microstructured poly(hydroxyacetic acid), polyglycolide (PGA). This material is able to keep the pH of a surrounding solution within the physiological range (7.2–7.6). This was achieved by chemical fine‐tuning of the counterplay between the acidic degradation of the polyester and the basic dissolution of calcium phosphate. Microporous samples with pore sizes of <1 μm and compact samples were prepared. The biological behavior was assayed in vitro by long‐term osteoblast culture. Morphological and biochemical analyses of cell differentiation revealed excellent biocompatibility, leading to cell attachment, collagen and osteocalcin expression, and mineral deposition. This material could be of use as a biodegradable bone substitution material and as a scaffold for tissue engineering. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res 54: 162–171, 2001