Smooth, perfectly spherical, highly hydrophilic microspheres have been prepared from crosslinked poly(methyl methacrylate) microspheres by alkaline hydrolysis in ethylene glycol at elevated temperatures. These microspheres absorb varying quantities of water depending upon the extent of hydrolysis. Subcutaneous implantation studies on rabbits demonstrated that the microspheres are biocompatible. Implantation studies in the renal arteries of dogs demonstrated the occlusion effect produced by the microspheres. Microspheres could be made radiopaque by the incorporation of barium sulphate. Potential uses envisaged for these microspheres in the biomedical area are that of artificial emboli for endovascular embolization and as microcarriers for the growth and propagation of anchorage dependent mammalian cells.