The present invention relates to an assay for free protein S comprising the addition of a ligand specific for free protein S to a biological fluid sample to form a protein S/ligand complex, and subsequent determination of the amount of the complex formed in the sample. The ligand specific for free protein S is comprised of the C4b binding protein (C4BP), or part thereof, or a compound comprising an amino acid residue sequence that binds specifically to the binding site for C4BP in protein S. The present invention further relates to antibodies specific for free protein S, which can be used as ligands in the assay of the invention, and with protein S related polypeptides, which can be used to produce such antibodies. In addition, the present invention is related to diagnostic test systems, suitable in kit form, comprising the present ligand and at least one further reagent required in the assay for free protein S.