Human haptoglobin type Hp 1-1, is hydrolyzed by plasmin to give two dissimilar products, P1 and P2, with molecular weights of 78 000 and 17 000, respectively. The larger fragment, P1 has the same N-terminal amino acids, valine and isoleucine, as intact haptoglobin. The smaller fragment, P2, is derived from the carboxyl-terminal ends of the β-chains, and has an N-terminal serine.The haptoglobins of genetic types Hp 2-2 and Hp 2-1, which are found as a series of oligomers, are also sensitive to digestion by plasmin in an analogous manner.