The primary structure of an elastase inhibitor located in the cytoplasm of leukocytes obtained from the equine species has been determined. By sequence comparison, the protein was found to be a member of the serpin family with strong identity to plasminogen activator inhibitor-2. In contrast to other serpins this protein contained no carbohydrate and had a blocked amino terminus. Preliminary evidence indicates that the inhibitor has the additional feature of being a thymosin beta 4-binding protein, since this polypeptide was always located in purified preparations of the protein. This suggests a physiological role for cytoplasmic elastase inhibitors in the thymosin beta 4-regulated rearrangement of the cytoskeleton of leukocytes.