Antiflammin 2 (HDMNKVLDL, AF2) is a synthetic peptide derived from the region of highest sequence similarity of lipocortin I and uteroglobin, and is a potent antiinflammatory agent without any known side effects of corticosteroids. The antiinflammatory activity of AF2 has been demonstrated, but is not reproducible between laboratories. It has been suggested that the chemical instability of this peptide is responsible for the loss of activity. The degradation of AF2 in aqueous solutions at a pH range of 3 to 10 has been reported. In this study, the degradation of AF2 at acidic pHs was monitored by reversed-phase HPLC. The reactions were studied as functions of buffer concentration and temperature. The rates of loss of AF2 followed apparent pseudo-first-order kinetics. Several products were isolated and identified by fast atom bombardment mass spectroscopy and tandem mass spectroscopy, and were the result of C- and N-terminus hydrolyses of aspartyl peptide bonds in AF2. The peptide bonds at C-termini of the aspartyl residues were most susceptible to hydrolysis, resulting in the formation of major degradation products, HDMNKVLD, MNKVLDL, and MNKVLD. The minor products from the N-terminus hydrolysis were HDMNKVL and MNKVL and formed at much slower rates.