The aim of this study was to identify two homologous serine proteinase inhibitor (serpin) molecules, squamous cell carcinoma (SCC) antigen-1 and -2, by two-dimensional electrophoresis (2-DE), combined with immunoblotting, and examine their expression in tumor tissue. The recombinant SCC (rSCC) antigen-1 showed four spots with p/ 6.5, 6.4, 6.3 and 6.0, whereas rSCC antigen-2 showed a more acidic spot with p/5.95. SCC antigen in tumor tissue appeared in three new acidic spots (p/5.7-5.5, M(r) 44 500), numbered 5, 6 and 7, besides the previously reported four spots numbered 1 to 4. These new acidic spots of SCC antigen apparently increased in SCC tissue. Treatment of tissue extract by carboxymethyl (CM)-papain agarose matrix extinguished spots 1 to 4 encoded on the SCCA1 gene, but not 5 to 7 on the SCCA2 gene. Overexpression of the SCCA2 gene may play an important role in the malignant behavior of tumor cells.