Intercellular spaces between adjacent mucosal epithelial cells are sealed by tight junctions (TJs) that prevent the free movement of solutes across the epithelium. Claudins (CLs), a family of 27 integral membrane proteins, are essential components for TJ seals. We previously used a CL-3/-4 binder, the C-terminal fragment of Clostridium perfringens enterotoxin (C-CPE), to show that CL modulation is a promising method to enhance mucosal absorption. Recently, by using a C-CPE mutant library, we developed a CL binder (m19) with broad specificity to CL-1, -2, -4, and -5. Here, we compared the mucosal absorption-enhancing activity of C-CPE and m19. Both CL binders enhanced jejunal absorption of dextran with a molecular mass of 4000 and 150,000 Da and nasal absorption of dextran with a mass of 4000 Da but not 150,000 Da in rats. Although both binders showed similar nasal absorption-enhancing activity of dextran (4000 Da), m19 exhibited a more potent jejunal absorption-enhancing effect than that of C-CPE. These findings suggest that mucosal absorption-enhancing activity may be modified by modulating CL specificity. (C) 2012 Elsevier Inc. All rights reserved.