A plasminogen-binding site of human alpha 2-plasmin inhibitor was studied. The chromatogram of digest from the amidinated alpha 2-plasmin inhibitor (67K-daltons, plasminogen-binding form) with trypsin was almost identical with that obtained from the 65K-daltons derivative (non-plasminogen-binding form) treated with the same procedure, except for the three tryptic peptides. One of the three peptides, the deamidinated peptide T-11, was found to have a strong ability to inhibit the interaction of alpha 2-plasmin inhibitor with human plasmin. Moreover, the dissociation constant Kd for interaction between the peptide T-11 and plasmin was estimated to be 5.5 microM, indicating that Kd is about 10-fold lower than that of epsilon-aminocaproic acid. The sequence of the peptide T-11 was determined by the Edman method as follows: NH2-G-D-K-L-F-G-P-D-L-K-L-V-P-P-M-E-E-D-Y-P-Q-F-G-S-P-K-COOH. alpha 2-Plasmin inhibitor and its 65K-daltons derivative were found to have the same NH2-terminal sequence of Asn(Asp)-Gln-Glu-Gln-. These results indicated that the plasminogen-binding site(s) of alpha 2-plasmin inhibitor could be located in the COOH-terminal region of its molecule and that some of epsilon-NH2-groups in the deamidinated peptide T-11 may be involved in the lysine-binding site(s) of plasmin(ogen).