C-Reactive protein (CRP) and serum amyloid P component (SAP) are two closely related proteins with respect to their primary structure and their pentameric appearance under the electron microscope. The two proteins have unknown functions. A common property shared by CRP and SAP is their ability to bind to sulfated polysaccharides and to fibronectin in a calcium- dependent manner. Using techniques including cell attachment assays, tissue culture, peptide synthesis and immunoassay, a peptide modeled after the primary sequence of SAP was found to bind strongly and specifically to heparin and certain other sulfated polysaccharides. This binding was independent of calcium. The homologous peptide from CRP also bound heparin. When native SAP was denatured or digested with tyrpsin the heparin binding property remained, but was no longer calcium- dependent. Thus calcium may influence the conformation of SAP to expose the active peptide region. Consistent with this suggestion is the finding that the synthetic peptide competes with the native protein for heparin.