The aim of this study was to detect the cleaved form of serine proteinase inhibitor (serpin), squamous cell carcinoma (SCC) antigen-1 in normal and malignant squamous epithelial tissues, which implies the presence of its target proteinase. The cleaved SCC antigen-1 in normal squamous epithelium was identified as a single spot with pI 6.35 and M(r) 40,000 by two-dimensional electrophoresis (2-DE) combined with immunoblotting. Interestingly, the cleaved form showed different biochemical properties in heat stability or immunoreactivity with a monoclonal antibody for SCC antigen (Mab 426) compared to intact SCC antigen-1. Furthermore, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of tissue extracts showed an abundant 40 kDa band of cleaved SCC antigen-1 in tumor tissue compared to normal tissue. Among the potential target proteinase of SCC antigen-1, immunoblotting analyses revealed that cathepsin L2 was remarkably overexpressed in tumor tissue, while cathepsin L was expressed in both normal and tumor tissues. These findings indicate that SCC antigen-1 interacts with specific endogenous proteinases such as cathepsins L and L2 in physiological and pathological states of squamous epithelium.