Antimicrobial peptides are promising novel peptide leads, but their low serum stability often limits their further consideration in drug development programs. Here, we describe a generally applicable strategy to stabilize peptides against serum proteases by replacing arginine residues with alpha-amino-3-guanidino-propionic acid (Agp). Peptide NH(2)-RRWRIVVIRVRR-CONH(2) was nearby totally degraded after 8 h in mouse serum, whereas the variant with Agp substituted was degraded less than 20%. The antimicrobial activity was not affected.