Human alpha 2-antiplasmin was digested with CNBr and the partial NH2-terminal amino acid sequences of nine purified fragments were determined. One of these sequences (12 residues) revealed homologies with that of residues 158 to 168 and with that of residues 316-327 in the antithrombin III sequence and with the corresponding sequences in alpha 1-antitrypsin and ovalbumin. One CNBr-fragment of 69 residues with blocked NH2-terminus was subdigested with trypsin and chymotrypsin and most of its sequence aligned. This peptide probably represents the major part of the previously reported COOH-terminal fragment of alpha 2-antiplasmin with Mr 8,000 (1). In this sequence no clear homology with antithrombin III, alpha 1-antitrypsin or ovalbumin could be detected.