The levels of alpha 1-proteinase inhibitor (alpha 1-antitrypsin) in keratoconus, normal human, and other diseased corneas were examined. Using an immunoperoxidase technique, the presence of this inhibitor was demonstrated in the epithelium, stroma and endothelium of all corneal sections. Compared with normal human controls, the staining intensity in the epithelium and stromal lamellae of keratoconus corneas was markedly reduced. Such a reduction was not seen in either scarred or other diseased corneas. Extracts of keratoconus and normal human corneas were subsequently analyzed for alpha 1-proteinase inhibitor by a dot blot assay using a monoclonal antibody against the inhibitor and a 125I-labelled secondary antibody. In agreement with the immunohistochemical findings, the alpha 1-proteinase inhibitor level found in the epithelium of keratoconus corneas was approximately one-fourth of that found in normal human controls. In addition, the stromal extracts of keratoconus corneas contained about one-sixth the inhibitor level of that in normal human extracts. These results lend further support to the hypothesis that degradation processes may be aberrant in keratoconus.