Recently, tumor markers, which have been developed for the diagnosis and evaluation for cancer, are regarded as new key tools to study the malignant behavior of cancer, and therefore, much attension has been focused on their biological functions. Squamous cell carcinoma (SCC) antigen is a tumor marker of SCC arising in various sites. We have investigated the biological function and heterogeneity of this tumor marker by electrophoretic methods. In this paper, we report that SCC antigen belongs to the inhibitory type of serine protease inhibitor family, indicating that SCC antigen may affect malignant behavior such as tumor invasion. We also describe the different pattern of SCC antigen by two-dimensional electrophoresis in normal and cancer tissues, and indicate that the heterogeneity of SCC antigen results mainly from the presence of two homologous genes, SCCA 1 and SCCA 2 gene.