Three monoclonal antibodies identified as D8, B11 and C5 of different specificities have been produced against human antithrombin III (AT). The apparent dissociation constants (Kd app) of the AT-antibody interaction were determined by ELISA method: Kd app (D8) = 2.4 nmole, Kd app (B11) = 13 nmole, Kd app (C5) = 24 nmole. All three antibodies reacted with isolated AT on immunoblots obtained with "native" PAGE. The D8 antibody also reacted with plasma and serum AT while B11 antibody reacted with serum thrombin-antithrombin (TAT) complexes as well.