Abstract: Disclosed is a procedure for conducting anti-thrombin III assays in which the conventional thrombin-anti-thrombin III treatment step is conducted at a pH of 7.9 to 8.5 maintained at the level by a buffer system comprising a buffering amount of N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid, N-2-hydroxyethylpiperazine-N'-3-propanesulfonic acid, N-tris-(hydroxymethyl)-methyl-2-amino-ethanesulfonic acid, N-tris-(hydroxymethyl)-methylglycine, or N,N-bis (2-hydroxymethyl)-glycine or a salt thereof. The compositions may also contain bacteriostatic agents.