A composition for controlled release of a biologically active agent from a carrier. The biologically active agent is heparin or a related biologically active acidic polysaccharide and the carrier is a sol-gel derived silica xerogel. The xerogel is derived from a tetraalkoxysilane such as tetrethoxysilane (TEOS) and part of the tetraalkoxysilane is replaced by an organomodified alkoxysilane, preferably an akylsubsitituted alkoxysilane. The invention also includes a method for the preparation of this composition.