A monoclonal antibody (MAb/T2G1s) was prepared by fusion using spleen cells from mice immunized with the NH 2-terminal CNBr fragment of human fibrin II, the so-called (T) N-DSK [(Aα 17–51, Bβ 15–118, γ 1–78) 2]. In competition experiments, this antibody reacted with (T) N-DSK as well as peptide Bβ 15–42 which can be obtained from (T) N-DSK by digestion with plasmin. Little or no reaction was observed with intact fibrinogen, the NH 2-terminal CNBr fragments from fibrinogen ( N-DSK) or fibrin I [(B) N-DSK], respectively, as well as peptide Bβ 1–42. These results suggest that MAb/T2G1s is directed to an epitope on the Bβ chain in fibrin II but not in fibrinogen or fibrin I. As such, MAb/T2G1s differs completely from another antibody (MAb/1-8C6)—also specific for the NH 2-terminal region of the Bβ chain—which was recently described [Kudryk et al. (1983) Molec. Immun. 20, 1191–1200].