Human alpha 1-antichymotrypsin, isolated at pH 8.0 from both normal and acute phase plasma, has been found to have two different amino terminal sequences despite the fact that inhibitory activities are unchanged. In normal plasma over 90% of the protein has an amino terminal sequence beginning with aspartic acid and less than 10% with arginine. However, in acute rheumatoid arthritis plasma 55% of the inhibitor begins with arginine and the remainder with aspartic acid. Sequence studies indicate that a fifteen amino acid peptide fragment has been cleaved to yield the arginine protein. Human alpha 1-proteinase inhibitor also shows this heterogeneity, but the ratios do not change between normal and acute phase plasma. It may well be that the missing peptide has some biological activity manifested only in the acute phase state.