Ovalbumin, the major protein in avian egg-white, is a non-inhibitory member of the serine protease inhibitor (serpin) superfamily. The crystal structure of uncleaved, hen ovalbumin was solved by the molecular replacement method using the structure of plakalbumin, a proteolytically cleaved form of ovalbumin, as a starting model. The final refined model, including four ovalbumin molecules, 678 water molecules and a single metal ion, has a crystallographic R-factor of 17·4% for all reflections between 6·0 and 1·95 Å resolution. The root-mean-square deviation from ideal values in bond lengths is 0·02 Å and in bond angles is 2·9°. This is the first crystal structure of a member of the serpin family in an uncleaved form. Surprisingly, the peptide that is homologous to the reactive centre of inhibitory serpins adopts an α-helical conformation. The implications for the mechanism of inhibition of the inhibitory members of the family is discussed.